Just a wild guess:

Document menu > Document Options > Multimeasure Rests

Is the Multimeasure Rest option

Update Automatically

selected?

If the answer is ‘yes’, then de-select it - and you will probably be “back in business”.

Explanation:

The option {Update Automatically} is useful in the initial phase when you are entering the music (probably from Scroll View).

As music is entered in an empty measure inside a multimeasure rest, {Update Automatically} will break the multimeasure rest, and reset all measure widths to the default.

This also happens automatically when you go to the Print dialog.

When you have finished entering music, and are working on the layout, you can safely de-select {Update Automatically}.