Hey there SlippySlope,
Thanks for reaching out to Apple Support Communities. I understand the display arrangement resets every time you restart your Mac. I’ll be happy to help with this.
If you haven’t already, reset the SMC and NVRAM of your Mac:
How to reset the SMC of your Mac
Reset NVRAM or PRAM on your Mac
After you reset the NVRAM, your Mac should boot up as normal. Arrange the displays as needed and then boot into safe mode and see if the issue returns there:
How to use safe mode on your Mac
‘If the issue continues while in safe mode, or your Mac restarts several times and then shuts down while starting up in safe mode, you should reinstall macOS and make sure that your Apple software and third-party apps are up to date.
If the issue doesn't continue while in safe mode, leave safe mode by restarting your Mac normally. Then test for the issue again. The issue might be resolved.
If the issue returns after leaving safe mode, startup items are probably at fault. Apps can install such items in numerous places, and they aren't always easy to find or recognize. For that reason, you should first make sure that you've installed any available updates for your Apple software and third-party apps.
If updating your software doesn't help, try any of these solutions:
Regards.
