Musescore 4.5 - Issue with score spacing - stretch not working?

I've been using musescore 4.5 for a while, and generally very impressed. However, just hit an issue with attached score.

I want to reduce the width of the 1st 2 bars (after pickup bar) but nothing I do makes any difference. If I select them both and use "reduce stretch" nothing happens. If I select the 1st three bars and use "reduce stretch" all that happens is that bars 1 and 2 reduce in width but the pick up bar just gets wider and wider to compensate, and nothing happens to bar 3. If I select the entire score and reset stretch, nothing happens. If I select multiple bars and try to set the stretch manually, nothing happens. If I select one bar at a time, and manually change stretch this does change the stretch of that bar. However, when I move on and manually select the stretch of the next bar, this undoes the effects on the bar before.

I have checked, and there is no hidden line break at the end of the 1st line. I've applied "reset stretch" to the attached file so every bar now has a nominal stretch of 1.0.

Nothing I do seems able to affect the 1st 2 bars!!

Any ideas anyone???