Since the introduction of version 3.0.6 any Force reactivation sets the last Active date to a dash
I am advised that this behavior changed from 3.0.5 -> 3.0.6 because of this revision (r10103) in an attempt to fix
PHPBB3-8318. It was recommended to me that I submit this to tracker.
I have also been told that it happens with account deactivation or force reactivation.
The date last active should not be removed on account deactivation and force reactivation.