[Shift + ←] causes wrong visuals on notes.

Bug report for https://musescore.org/en/node/310744 and https://musescore.org/en/node/310759
Not sure what the title should be (or what causes the bug), so I just went for the one you can see right now.

What I did / how you can reproduce this:
1. Create the score shown in the fist picture:

2. Click on the last note
3. Click [N] to enter some special mode
4. Click [Shift + ←]
5. The Measure should now look like the one shown in the second picture:

As far as I'm aware, the second measure now is longer than 4/4 because the last note turned into a quarter note and nothing else changed. However:
1. There's no floating "+" icon that shows me that the measure's too long
2. The rhythm still sounds identical.

What is happening? Does this high quarter note mean something different?
EDIT: This also works with a measure as simple as this one: