Score created in 2.3.2 opens badly in 3.0.2

When I open a score created in 2.3.2 into 3.0.2, it appears to open correctly.
However, if I then try to do a transposition, I get key signatures appearing mid system.
Here a partition created in G transposed to Ab :

If I then try to select the key signature, it shows that it is selected, but I cannot remove it.

Going further, if I export the score to musicxml, it shows that at measure 11 defines a new system - which was not defined manually in the original score created in 2.3.2

Is this a known problem ?
Is there a workaround to get over this ?

I have not kept the original score pre v3.0.1 opening, unfortunately, but am including the one which shows this behaviour.