Deleted notes one by one in bar do not result in a full rest
S4 - Minor
When notes are deleted one at a time from a 3/8 time measure, the last note deleted should result in a full measure rest. However, the bar either contains three one-eigth rests or a dotted one-quarter rest, depending on what notes were originally present and which were deleted in which order.
I have discussed this on the forum at http://musescore.org/en/node/13617
I think this is by design. Meaning, you might want to be able to have a 8th rest and a quarter rest in the measure, for example to change the second rest to a note later. If MuseScore was creating the full rest automatically for you, you'll not be able to do this. If you do want a full measure rest, just select the measure and press Del.
Fixed in branch master, commit c7e9144fbc
_Merge pull request #15001 from asattely/cross-measure-beam-secondary-break
Fix #13624: allow BeamMode::BEGIN32 and BEGIN64 to continue cross-measure beam_