Hi Yixing, thank you for your bug report. This issue has already been reported, and the discussion is in issue #22377.

In the short term I believe the solution will be to document the current behavior. In the long term there are some solutions, though I imagine none of them will be amazingly satisfying, because the output of %Z is basically freeform and doesn't necessarily match to an unambiguous offset. The best you'd be able to do would be %Z%z or %z%Z, but there will be many implementation challenges there.