I have an Android phone and it is currently stuck in desktop mode for e621. This has happened before and would randomly fix itself to my knowledge. This time I decided to delete the cookies and it caused it to sign me out which reverted to app mode but every time I signed in would change it back to desktop mode.

Have you tried clicking the button on the bottom right of the page that says "Disable mobile mode support" or "Enable mobile mode support"?

Oh shit, thanks a lot!