I may not have found the correct place to look, but based on my initial searching, I don’t think this is a Discourse bug.
Discourse does not seem to apply any bindings to CTRL+Z, so it is letting the browser handle it and it seems each browser sucks at it in various degrees (some are REALLY bad and others mildly horrendous).
(just noticed @Falco moved this to UX, which seems to support my findings)