in the case of HST (or whatever the successor issue is now), i think that’s addressed in a separate bug report: https://forum.inaturalist.org/t/account-time-zone-sometimes-automatically-sets-itself-to-hawaiian-time/11940.
in the case described in this thread, the time zone is ADT. i suppose it’s possible that ADT an earlier version of the HST problem, but i don’t see a lot of cases of ADT. so i assume it’s unrelated. even if it’s related, the issue is already covered in the HST bug report.
regarding UTC, i see that you mentioned you had a lot of observations set to UTC (https://forum.inaturalist.org/t/large-number-of-app-users-with-date-observed-missing/11498/59), but i don’t remember if anyone had actually worked out what causes UTC, other than perhaps someone defaulting their account setting default to UTC?