Wrong Page Margins

MuseScore gets it defaults page size from the default paper size of your default printer, so it must be set to something non-standard. If changing that isn't convenient, then you can create a style file (MSS) with your own preferred sizes. Just set up a score as you like, then go to Format / Save Style, specify a name to save to, then edit the file to remove everything note related to page size. Then you can specify that as your default in Edit / Preferences / Score.

If you continue to have trouble, it is best to ask for help on the Support forum and include more info, like what OS you are on, what your default printer / page size actually is, and a sample score you get if you simply create a new one using the Treble template and then immediately save, so we can understand and assist better.

I, too, have seen some anomalies with margin handling.
1. Often I use "ledger" for my score and "letter/ANSI A" for parts. When I first bring up the Page Settings for a part, the right margins are not set to a reasonable value. When I change a left margin, the corresponding right margin changes to a reasonable initial value.
2. I've tried to Save/Load Style for the parts. The margins are not loaded. I looked at the XML in the MSS file and the margins appear to be save - assuming that the odd-left equals the even-right, and vice versa. However, the margins do not load from the Style files. I created a style file with only the margins and the header/footer definitions. When I load that file, the "letter" page format reverts to the defaults except for the right margins which go to 0. See attached MSS file.
3. The Page Settings preview sometimes doesn't match the actual page. In these cases, the settings shows in Page Settings do not match the settings in the actual page. The effect is such that making a change and changing it back (so OK or Apply will work) changes the page. See attached MSCZ file in which I kept one part which demonstrates the problem.

The way MuseScore records page size & margin info is a bit odd. Instead of literally recording the page width, left margin, and right margin directly, instead what gets record is the pagePrintableWidth and the left margin, and the right margin is computer from that. So it's important an MSS file specify the pagePrintableWidth reasonably. I guess this has to do with the attempt to be able to make things work consistently between Letter and A4, but what it means in practice is, there are definitely ways for things to get messed up.

The preview in page settings not matching is a separate known issue that I'm kind of surprised doesn't get reported more often - it was the thing that originally made me assume we would need a 3.6.3 fairly quickly after 3.6.2. But somehow it never caught fire. See #317272: Page settings in parts are initialized incorrectly in dialog if using pre-3.5.2 defaults (which I notice you were one of the only people to have also commented on!)

I see I had commented on #317272 and submitted an example. Unlike my "Swing Lake" example, the example I submitted with this thread was not up-leveled - it was created with the 3.6.

Since complaints are usually proportional to the annoyance factor of the problem, I suspect that the lack of complaints about a problem is related to the likelihood of encountering the problem and the ease of working around the problem.
I found these problems relatively easy to work around. I suspect others did likewise.
Dealing with the margins beyond selecting the type of paper seems to me to be something that most users would not need. I think relatively few of us push MuseScore to its limits - they just want to put something on paper in a form that can be played, etc.
The rest of us push the limits because we want professional looking scores and parts. We are the ones who notice when something is only close to being correct. Sometimes we don't have the time to report those problems, so we make a note and, eventually, report it (or check the new version to see if it has been fixed).
Summary: easy to work-around and most don't encounter the problem, so few complaints.

So, my #3 is known.
I fixed my page width and cleared my problem (in #2) with the right margin being zero. However, the left margin is being ignored. Since all the other XML fields seem to work, is it possible the left margins are misspelled when being fetched from the XML?
My #1 might be another symptom of the margin problems in #2 and #3.

Thank you for your response.