Joining bars that include a time signature change has odd results

I have been writing a piece that alternates between 4/4 and 3/4. After entering the notes I decided it would be better if in some cases the 4/4 and 3/4 time signatures were the other way round. I tried joining the 4/4 and 3/4 bars to produce an effective 7/4 bar and then splitting that to produce a 3/4 bar followed by a 4/4 bar. However, what happened was not what I expected. I have recreated the operation (with a simplified score) with screen grabs after each step. (Note, I am using UK localisation and so "bar" = "measure")

1. Starting point

2. select the first two measures and use [Tools]>[Bar]>[Join selected bars]

Note that the time signature in the third bar has disappeared (but it still has 4 beats). Also, the 3/4 from the second bar is still there in the middle of the effective 7/4 bar.

3. select the F on beat 4 and use [Tools]>[Bar]?[Split bar before selected note/rest]

Now I have a 3/4 bar followed by a 4/4 bar as required but the time signatures need fixing.

4. Add a 3/4 time signature to bar 1, 4/4 to bars 2 and 3

Everything is OK except that I still have that unwanted 3/4 time signature in bar 2.

5. Select the time signature and press delete - nothing happens!
   Try CTRL+delete - nothing happens!
   Try CTRL+X, nothing happens!

To summarise, there are two odd things happening.
a) The time signature in the bar following the bars that were joined disappears (not such a big deal as it can be re-added.
b) The joining operation leaves an undeletable time signature in the middle of the joined bar.

Is this a bug, or am I pushing the boundaries of join/split operations too far?

OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.6.2.548021803, revision: 3224f34