Massive space on first page of score

I've just upgraded to musescore 4 (from version 3 after a laptop upgrade!). I'm doing brass band arrangements and it's been great, but I managed to get a corrupted file error where the top line had more beats in the bar than expected. I had to save anyway, and on reopening I deleted the contents of corrupted bars, re-entered the notes and the corrupted errors went away, so I thought all fixed.

However, I've gone back to the first page of music on the score (which is actually the second page as the first just has the title on), and there is a massive gap between the Bass Trombone and Euphonium staves, which as far as I know I didn't put there (see screen shot). I've tried manually changing the stave spacing but it has no effect. Have I found a bug or is there a way of fixing this?