Quality problems with adjustable vent-rite #1?

hi all- a few years back I replaced a lot of my non adjustable vents with what I thought was high-quality adjustable vent-rite 1#s. I recently had cause to remove some of the radiator covers and while the heat was on, I noticed that a couple of them have what appears to be hairline cracks. Did I just get unlucky or have others experienced this issue as they are not cheap and don’t want to spend upwards of $100 to replace these two if they are just going to fail again. Can they develop these cracks or likely that they were defective when I installed them?