If I create a couple of bars of music, the first of which has a Whole Measure Rest then a grace note, and the second has some "normal" music, all in a time other than 4/4, then the bar with the WMR and grace has the wrong time value - the WMR is treated as a true semi-breve rest, not a rest equal to a bar's length. This seems to me a bug - a grace note should not alter the interpretation of a WMR.
I have no opinion on the bug part. Here are a couple of ways to get what you want:
!NoteWorthyComposer(2.5)
|AddStaff|Name:"Staff"
|StaffProperties|EndingBar:Open (hidden)
|TimeSig|Signature:3/4
|RestMultiBar|NumBars:1|PrintOnce:N|WhenHidden:ShowRests|Visibility:Never
|Note|Dur:8th,Grace|Pos:-1
|Bar
|Text|Text:" l"|Font:StaffSymbols|Pos:2|Placement:AtNextNote
|RestMultiBar|NumBars:1|PrintOnce:N|Visibility:Never
|Note|Dur:8th,Grace|Pos:-1
|Bar
|AddStaff|Name:"Staff-1"
|StaffProperties|EndingBar:Open (hidden)
|TimeSig|Signature:3/4
|Rest|Dur:Whole
|Bar
|Rest|Dur:Whole
|Bar
!NoteWorthyComposer-End