I suspect that you are doing something wrong.

Please specify your steps.

What are your steps when changing a score to {page orientation landscape}?

What are your steps when changing an extracted part to {page orientation landscape}?

To be honest, I've tried so many different things at this point I can't even remember what I did. A fellow band director just told me to try:

Command + Alt + > to index through parts, then Document>Page Format>Parts and click landscape. That let me get it to Landscape mode, but when I do Document>Manage parts>Generate part it reverts it back to portrait mode again.

… changing an extracted part to landscape mode …

… Document>Manage parts>Generate part …

This is confusing.

To get linked parts, use

Document>Manage parts>Generate part

Linked parts are “inside” the score Finale document.

Correct a wrong note in the score, and the linked part is automatically corrected.

An extracted part is a separate Finale document, not linked to the score.

Correct a wrong note in the score, and the extracted part is not automatically corrected.

Are you talking about an extracted part, or about a linked part?

I was talking about an extracted part, but then my friend said "I don't extract parts anymore" and gave me what I typed previously. Even with his method, when I hit generate part, it reverted back to portrait mode. I'm just trying to create a part in landscape mode, and I don't really care how I get there at this point.

… and gave me what I typed previously. Even with his method, when I hit generate part, it reverted back to portrait mode …

According to your first post you are using Finale v26.3.1.643.

I launched my old Finale v26.3.1.643 - and tried to reproduce your problem.

Unfortunately I was unable to do so.

I opened a score document.

I changed the {Page Format for Parts} to Landscape orientation.

Then, I went to

Document menu > Manage Parts…

and clicked the button {Generate Parts}.

Then, I went to

Document menu > Edit Part > …

- and I got Landscape orientation in the linked parts.

The score remained in Portrait orientation.