The issue is that two MARIA_HA instances shares the same MARIA_STATUS_INFO
object during UNION execution, so the second MARIA_HA instance state pointer
MARIA_HA::state points to the MARIA_HA::state_save of the first MARIA instance.

This happens in
thr_multi_lock(...) {
...
	for (first_lock=data, pos= data+1 ; pos < end ; pos++)
	{
...
		if (pos[0]->lock == pos[-1]->lock && pos[0]->lock->copy_status)
			(pos[0]->lock->copy_status)((*pos)->status_param,
																	(*first_lock)->status_param);
...
	}
...
}

Usually the state is restored from ha_maria::external_lock(...):

\#0  _ma_update_status (param=0x6290000e6270) at ./storage/maria/ma_state.c:309
\#1  0x00005555577ccb15 in _ma_update_status_with_lock (info=0x6290000e6270) at ./storage/maria/ma_state.c:361
\#2  0x00005555577c7dcc in maria_lock_database (info=0x6290000e6270, lock_type=2) at ./storage/maria/ma_locking.c:66
\#3  0x0000555557802ccd in ha_maria::external_lock (this=0x61d0001b1308, thd=0x62a000048270, lock_type=2) at ./storage/maria/ha_maria.cc:2727

But _ma_update_status() does not take into account the case when
MARIA_HA::status points to the MARIA_HA::state_save of the other MARIA_HA
instance.

The fix is to restore MARIA_HA::state in ha_maria::external_lock() after
maria_lock_database() call for transactional tables.