When the measure widths automatically get changed, the culprit could be Automatic Music Spacing.

But the culprit could also be a document setting:

Document Options > Multimeasure Rests > Update Automatically

When - from Scroll View - notes are added to a multimeasure rest, the setting {Update Automatically} breaks the multimeasure rest, and adjusts the layout.

The setting affects the widths of all measures, not just the widths of multimeasure rests.

You should de-select {Update Automatically} when you have finished entering music, and begin working on the layout.

But you probably already knew that.

Thanks, and I agree with what you wrote, but I am not sure you understand my main problem. I don't care about creating multi measure rests. It is about manual measure width adjustments of any measure, including the width of multi measure rests. And the only way to keep them as of now is by never to space the music again. 

Yes, ideally, you do all the layout tweaking after all the music has been inputted. But how often do we make changes, add measures, change rhythms of just a few bars after a rehearsal for example. That's when it gets tricky. It would all be super easy if Final would allow us to lock our layout and unlock it for selected measures only. And of course it would really help to have my feature request for incorporating manual measure width adjustments when spacing music taken care of.

Cheers!

I have been asking for a "leave it alone, dammit" setting for a long time.

Another, similar request:

Make the setting {Automatic Music Spacing} a document setting, rather than a program setting.

Maybe I'm misunderstanding the problem. But all of my multimeasure rests which get respaced manually keep any adjustments I make, they never change on their own.

I make my spacing decisions at the end of the engraving process with, locking systems, turning off automatic music spacing and turning off multi-measure rests, update automatically. With these settings intact all multi-rests follow my spacing decisions.

Of course it keeps your manual measure width changes when you turn off automatic music spacing and you don’t space it manually. But we shouldn’t have to worry about that in the first place. If our measure width changes would be locked (which should be the default setting in my opinion), and any manual spacing gets incorporated when spacing the music, we would never have to worry about spacing the music after we made changes. You see what I mean?

In a related issue -- any expression (particularly tempo markings and other expressions which can include lengthy text) should have a setting available to force measures to be wide enough to accommodate them without having the expression extend into the next measure(s).