Not at all unlikely. I found that out the hard way but getting
shocked off the white wire. Feed to outside yard light runs
underground and has common white with another underground feed to a
shed. Circuit is non fixable without replacing the underground feeds.
At leas now, every box the two circuits run through are prominently
marked warning anyone working in them to pull -both- breakers.

Yes, it is unlikely; just not impossible as you found. Since the white it
grounded, it ought to be a great deal better ground than anyone touching it;
unless there is a problem, there should never be any voltage on it..

However, we are talking about the bare wire here, the grounding conductor.
Unless there is a problem there should never be any current on it, and
unless there is a second problem, there should never be any voltage on it.