Currently, when having objects with mixed timezones, they are each separately interpreted as their local time:
So both times are actually about the same moment in time (the same value in UTC; in pandas their stored value is also the same), but once converted to pyarrow, they are both tz-naive but no longer the same time. That seems rather unexpected and a source for bugs.
I think a better option would be to normalize to UTC, and result in a tz-aware TimestampArray with UTC as timezone.
That is also the behaviour of pandas if you force the conversion to result in datetimes (by default pandas will keep them as object array preserving the different timezones).