Help! Mystery pilot issue in greenhouse propane heater

Hi! Newbie in need of help/advice, so sorry about the length, but I didn't know what to cut. Some background: I've had a greenhouse for just over 20 years. For the first 10 or so I used propane. Propane became a hassle for various reasons, so then I used electric until I upgraded to a larger, better insulated greenhouse last summer. In January I got a new ventless propane heater (BTU output recommended based on surface area, insulation factor and area's low temps), new tank regulator, hose, etc. and got my 100 and 40lb tanks filled up. I'd built in adjustable fresh air ventilation on each end of the greenhouse (one at top, one a bottom) to allow for a higher BTU heater (Arkansas winters have become pretty brutal lately, with consecutive days, even weeks in single digit, sub-0 temps).

Pilot wouldn't stay lit. I checked for drafts, oxygen depletion (as best I could with a carbon monoxide sensor), leaks, so forth. Manufacturer wasn't helpful and only suggested maybe it was too much BTU and I'd probably need to leave the doors opened. Didn't make a lot of sense to me since it had ample fresh air and was only ever on the lowest heat setting, but I returned the heater and replaced it with a DynaGlo 12k BTU ventless propane wall heater (I purchased legs) and got that set up in February.

Same issue. As far as I could tell (I have smart temperature sensors and their app), heater was fine for a day or three, then on the afternoon/evening it would go through 3 or 4 cycles, then not come back on. Being a stubborn old woman, I sat and watched to catch it in the act. The pilot would simply snuff out 15-20 minutes after it cycled off. Never had a problem relighting.

So, I bought a second regulator, no go; tried various venting configurations, fans, I even used a fog machine to detect air movement and make sure there WAS plenty of airflow (even though DynaGlo manual states the room size for 12k would not be considered constricted); checked for leaks, obstructions, pilot/thermocouple positioning; googled for hours; called manufacturer (who was again no help since a licensed professional didn't install it). Finally returned that for same model.

Same issue. So now I'm thinking it's not the heater, or the regulator (had two new ones at this point). Called a professional. Apparently pressure on the "preset" regulator was too high, so he adjusted it. Used 5-6 weeks till end of season, no problems! (I was SO happy).

Fast forward to last month. Fill tanks, dust off heater, reconnect and check everything, fine for a week or so, then same issue, or I should say, same result (pilot out).

I videoed the normal pilot after burner cycle (attached); this was very similar to issue in February, where part of the flame at the tube would "disappear," then it would simply go out.

But instead of simply going out, it's like a high velocity something was acting on the pilot to the point where the flame "separated" from the pilot tube, and reappeared on the other side of the thermocouple (also attached); and there was a "high velocity" sound. I'm assuming the pilot goes out because the thermocouple isn't heated. It does this for approx 35-40 seconds before going out.

So, re-did most February's troubleshooting, (sans buying yet another tank regulator and using the fog machine), before calling same professionals as before. Different guy came out, admitted to not being too familiar with smaller setups. Said tank regulator had zero pressure, likely has ants in it. I noted there no evidence of ants (or debris of any kind) on the regulator or any where in the area (but what do I know). Opened the tank without the regulator to show me the tank had pressure -twice. Tried the second new regulator, said that had pressure. I saw analog gauge at 15, and assumed that was water columns, I asked if it needed to be adjusted to 11. He said you can't adjust them, they're sealed. I told him in February they resolved by adjusting the other one, but didn't fight him on it, figuring with my luck, it's not unreasonable to have a regulator with barely a couple hundred to just go "bad" (even though we'd just had the heater on before testing pressure). He said to watch and if it happens again, call him and he'd come out again a bring another regulator.

It did happen again, later that afternoon. Three cycles, then 20 minutes or so later, pilot would blow off. Finding it hard to believe a regulator with a couple hundred hours on it and another new regulator with maybe one hour on it were both bad, I called to see if same professionals could look up my ticket from February, blah blah. Finally got ahold of someone three days later, explained everything again, and he said he felt bad, but he didn't know what to tell me. They don't carry small heaters anymore because they have so many pilot issues technology has made them "too safe."

He said he'd look at videos if I'd send them, I did, but haven't heard back, suspect I won't.

I've since blown out the connection hose, took the front off the heater to see if something weird was blocking something. I ordered a manometer, since maybe this would be handy, and I still question the original regulator "going bad" after guy got there. When I went to check pressure on the original regulator, I find the gauge port appears to be sealed off on that one (pic attached, side question -is it? And if so, how do you check pressure without access to it?)

Before I give up and concede that physics are different at my house (entirely possible), and it's all been a waste of time, money and energy, I'm hoping someone might recognize my description and images of symptoms as something that makes sense. Could it be the tanks? The fuel itself? Something else?

Again sorry for length, and appreciate the opportunity to...um ..vent.