Helen as answered the specific problem, just adding a some
notes...

IBO switches modes when a control is focused. When focused
any defined editmask is used, when not focused any defined
displayformat is used. Things like date/time fields have
automatic displayformat and editmask values that are used
by default if no specific values are defined.

There was a change several months ago that effected date/time
fields - where the displayformat default value is used even
when an editmask is defined. This did not used to be the case,
previously a definition of editmask would override the defaults
for displayformat as well (you could still explicitly define a
separate displayformat if desired).

I was going to ask Jason to back this out, but never found the
time to check through it in detail (and so I guess its probably
too late now). It was a problem to me, because the enhanced
masks in my Enh controls were quite capable of handling the
displayformat requirement, now I need to define both
displayformats and (enhanced) editmasks to make my controls
work consistently.

The point of this little note is that you may find that you
need to define BOTH an editmask and a matching displayformat to
get the desired consistent result for date and/or time fields
in both focus and non-focus modes.

But, as Helen said, your particular problem is that '_' is the
unfilled character indicator value in your mask, so of course
that is what the VCL mask processing will display when the mask
is used (which is only when the control is focused).

--
Geoff Worboys
Telesis Computing