Huh. I wonder if this is why I'd sometimes get random corruption on my laptop's SSD. I'd reboot after a while and fsck would find issues in random files I haven't touched in a long time.

If you're getting random corruption like that, you should replace the SSD. SSDs (and also hard drives) already have built-in ECC, so if you're getting errors on top, it not just random cosmic rays. It's your SSD being extra broken, and doesn't bode too well for the health of the SSD as a whole.

I bought a replacement but never bothered swapping it. The weird thing is the random corruption stopped happening a few years ago (confirmed against old backups, so it's not like I'm just not noticing).

It's quite possible. Some SSDs are worse offenders for this than others. I have some Samsung 870 EVOs that lost data the way you described. Samsung knew about the issue and quietly swept it under the rug with a firmware update, but once the data was lost, it was gone for good.

Huh, I thought I got some faulty one, mine died shortly after warranty ended (and had a bunch of media errors before that)

I ran into this firmware bug with the two drives in my computer. They randomly failed after a while -- and by "a while" I mean less than a year of usage. Took two replacements before I finally realized that I should check for an fw update

Unless your setup is a very odd Linux box, fsck will never check the consistency of file contents.

It found problems in the tree - lost files, wrong node counts, other stuff - which led to me finding files that didn't match previous backups (and when opened were obviously corrupted, like the bottom half of an image being just noise). Once I found this was a problem I've also caught ones that couldn't be read (IOError) that fsck would delete on the next run.

I may not have noticed had fsck not alerted me something was wrong.

But metadata is data too, right? I guess the next question is, would it be possible for parts of the FS metadata to remain untouched for a time long enough for the SSD data corruption process to occur.

A ZFS scrub (default scheduled monthly) will do it.