Page size should always be written to the .mscx file
MuseScore's installed templates all omit the page size tags so that the new scores created from them will be in the user's default page size, which can vary. These templates are the only files that should work this way. All user documents should specify the page size so that they do not open in another user's default page size.
The solution is to always store the page size in the .mscx file, except when
MScore::saveTemplateMode is true. I will fix this issue in an upcoming PR. I wanted to create the issue now, while it is fresh in my mind.
This issue was discussed in detail on the developers' chat, ending here: