I have a totally vanilla test instance of 0.19.1; when I report a new issue, the Resolution is set to "open" automatically and there is no field on the page that allows me to override that. Later, when working with the issue, on those pages that allow changing the Resolution field, a different value can be selected but there is no ability to clear the field. So I don't know how you got an empty Resolution field.

Having said that, it's possible to resolve an issue with a Resolution value that makes no sense for a resolved issue (like 'open' or 'reopened'), and it's possible to resolve an issue as 'fixed' without specifying the version containing the fix.

In a vanilla installation, it's possible to take an issue directly from assigned to closed without an intermediate resolve step. The close page allows the Fixed in Version to be specified, but not the Resolution, so any issue closed this way will not appear in the Change Log. Because of the ability to close issues immediately, I think it makes sense to provide the chance to supply the same values as at resolve time. This would mean displaying Resolution and Duplicate ID fields in bug_change_status_page.php when the status is being changed to closed.

Other nice tweaks in this area would be to (a) prevent resolving/closing an issue with a value (open, reopened, etc.) that makes no sense for a resolved issue, and (b) require a fixed in version value if resolving/closing with a resolution value of fixed (if versions have been defined).

This is possible by adding a custom function. See 0004138.

Sorry to cause confusion. When I said 'not filled in' I actually meant that it remains at the default value 'still open'.
Since the Changelog only reports bugs that are
a) closed and
b) have resolution FIXED

My problem is that it is not enforced to set the resolution to 'FIXED' when you close a bug; this is an optional step that, when you skip or forget it, will leave Resolution at the default. I didn't mean to say it was ever 'empty'.

How are you closing issues? In a vanilla installation, when you change the status to "resolved", the default setting for the resolution field is "fixed". You can change it in a dropdown menu there. The "fixed in version" is also there, but has no default.

He could be closing the issue directly from new or assigned without ever changing the status to resolved. On a vanilla installation, report a new issue, then change the status to closed. You will not have the opportunity to set the resolution value, and you will wind up with a closed issue whose resolution value is open.

Ah, clear case of Mea Culpa here.
And it's got a bit to do with the way a lot of the workflow is hardcoded in Mantis...

I am not running vanilla Mantis. I severely recustomized the flow.
In our flow we move things from Build to Test to Closed.
And my 'resolved treshold' was at 'Closed', meaning you never see the field appear on a status change.
I moved the threshold back a few levels and voila, it works.

Ok, I remember again why I set the $g_bug_resolved_status_threshold to my endstatus...

I want the 'issues resolved' on the main screen (my_view_page.php) to show the completed, tested, closed, never-to-think about anymore bugs.
So I set the status too high, namely at our endstatus.
Sideeffect was that the resolved field dissapeared

In this case, I'm not sure you could make it 'mandatory' -

"Fixed in version" makes sense to set if the bug is actually fixed or gets fixed - however, there will be times where a bug might get resolved as a duplicate, no change required etc, and therefore it wouldn't be appropriate to set a resolution/fixed version.

Given this was initially reported against 0.19.1, if you feel otherwise, please reopen this bug (or a new bug) with changes required to be made in 1.2.17/1.3 that would still be applicable, and could easily be made.