What version of Mac OS are you running? And are you on an Intel or M1 Mac?
I’ve been on the Mac since 1984—every iteration of Mac OS from then to now (currently Big Sur v11.5 on M1 Mac Mini). I can tell you that—in keeping with your own experience—comments added via Get Info have always disappeared after restart. This has been (up to now?) a perennial bug. Over the decades, as different versions of Mac OS X were released, I kept trying to use Comments, to see if this had been fixed, but always to no avail. So I gave up on Comments a long time ago.
HOWEVER. . . I tried again today and—lo and behold!—the comments remained after a restart. Go figure. I’ve not tried to use Comments for several past revisions of Mac OS X, so I cannot offer an opinion as to when this bug might have gotten fixed, but it appears to be working with my current configuration (which is also on the M1 chip).
Perhaps it’s been fixed as of OS 11? Is the fix related to the M1 chip? I can’t say for certain. I will keep an eye on it now. So far today, comments have remained after a restart. I hope they don’t disappear at some point in the future.
NOTE 1: My experiment today has been via Restart only, not Shut Down and restart. I don’t know if that would make any difference. I don’t know why it would, but I will experiment with that later.
NOTE 2: Prior to the M1 chip, it was possible on Intel Macs to reset Parameter RAM (PRAM) on restart. On M1-based Macs this is no longer done. I mention this because (a distant memory suggests) it may be that there was some relationship between System Parameters and the inexplicable disappearance of Comments. That’s not to suggest that comments would disappear only if PRAM was reset on restart, but perhaps instead apart from that, for some reason; I don’t know.
My point (above) is that the bug may have to do with any version of Mac OS running on the Intel chip. Thus the problem might still exist on Intel Macs running Mac OS 11.x while, at the same time, not on M1 Macs running OS 11.x.
I would be interested to know of others’ experience with this.
JH - Idaho - USA