If I display my score in Page View as Single Pages Vertically, I quite often loose the focus on what I am doing. The score jumps back to page 1, even if I am working f.e. on page 67…
Funny thing: this weird behaviour I do only know from one other application called Sibelius (notation program) 
So either it is me, or my computer/system software (macOS), or I don’t know…
This is a known bug.
A cursory search of the forum would have yielded results such as
and
thanks pianoleo, I will read those posts. Strange though that it happens in Sib too…