Ahhhh you let OneDrive "hijack" your system's Documents folder. I naively did the same thing on one of my Windows 10 systems, and it drives me so crazy. I spent a bunch of time on the phone with MS on how to unwind this, and bottom line is that they told me a bunch of stuff that originally didn't work at all, then different stuff seemed crazy so I gave up.

Unfortunately Planner calls Windows for the path to the user's "Documents" folder, and OneDrive modified this in the OS' settings. If you look in that OneDrive\Documents folder you should see the usual ACP stuff, Images, Plans, Logs.

However, once you save a plan anywhere, Planner should go back there afterward, even after a restart. Are you not seeing this?