Page Number Position
Page number position is bottom left on even numbered pages, and bottom right for odd numbered pages. This positioning is appropriate for two-sided printing. If you toggle 'Two sided' off, located at Layout, Page Settings, the position of the page number should be constant, such as bottom centre, or bottom right.
Comments
I think it is 'by design' and can be set by Style/General->Header, Footer, Numbers
to allow customization. See https://musescore.org/en/handbook/score-properties#header-footer