This bug is caused by cached THD pointer of some Item objects.

There are several kinds of Item objects which cache the THD pointer to try to
lessen the impact of current_thd. However, generated column expression is only
parsed and fixed once during open table first time. Moreover, the table is
cached and shared by all sessions. If the Items which cache THD pointer as
part of generated expression, obviously, such an generated expression can't be
shared by all sessions. If they were, it would result in invalid read and
memory leak.

Because there are only 3 Item objects which have effect on generated
expression. The solution is to remove to cache THD but use current_thd
instead.