Inference about leaking heat

Well you certainly are losing heat — most likely warm air rather than by conduction — into the attic. That is to be expected. Rockwool is a good insulator, but it isn't much in the way of a draught barrier. What interests me, though, is the increasing split on the 19th as compared to the 18th. What else was going on?

You really wanna see how bad the leakage & infiltration is, shoot the home with an infrared camera. Mad Dog

View from both inside and out. Small inexpensive IR devices hook to your phone.

I'm guessing the difference is due to varying wind levels. Wind blowing on one side of the house will positively pressurize the interior if it is reasonably leaky, as most old houses are. Our 100-year-old house has measurably worse heat loss on windy days. So a calm day wouldn't force as much warmed air out of the house and into the attic, etc, via air leaks, while a windier day would.

Also, by coincidence, I just went around our house the other day with a thermal imager, and I noticed that the air coming out of the attic vents was at around 44 degrees, on a day when the outside air temp was in the mid-30's, and the outer surfaces of the rest of the house were in that same range. So the air coming out of the attic was 10+ degrees warmer than ambient, and that was with 10" blown cellulose in the attic. And that was on a low-wind day. I'm sure it would be worse in higher winds.

We failed to do adequate air sealing in the attic before insulating, which I now recognize was a mistake.

Sealing the top plates is important, especially in balloon framing. You might find this comparison informative. One house had the top plates sealed; one did not. If I had read this before insulating our attic, I would have spray foamed the top plates first.

https://energysmartohio.com/uncategorized/a-tale-of-2-houses-low-hanging-fruit-is-poisoned/

Be very careful about sealing or insulating around any lights which are ceiling mounted. They can get very hot it they are too tightly insulated or sealed around.

In typical older plaster and lath houses, there is a gap at the top between of the wall and plaster. This creates a nice little channels to leak air into the attic. Unfortunately your batts won't slow any of this airflow down, you need to plug these holes with something solid like canned foam or with larger gaps, pieces of rigid insulation sealed in place with canned foam.

Stuffing batts of any kind won't work as they will simply just filter the air leaks and won't stop them, the exception is dense packed cellulose as that is pretty restrictive.

Would have been an easier fix before the insulation went in, but not too bad as you can pull the batts up in sections as you work and relay them once done.

A budget thermal camera can show you where the major hotspots are in the attic.

Smoke test unfortunately won't show leaks inside walls, especially if balloon framed, those can be quiet substantial. Generally if the outside walls feel cold (less than 60F if there is insulation in it), you have air flowing through your walls.

”Stuffing batts of any kind won't work as they will simply just filter the air leaks and won't stop them”

Not so; dense packed fiberglass is highly effective. But you need to really pack in there. Or it’s just a filter as @Kaos said.
Dense packed fiberglass is better for the fire block applications since you can stuff it by hand wherever required. Not the case with cellulose which has to be blown into a closed space.