Why does the default app get deleted so many times? I have to set the default app every time I start the Windows

Hello, thank you for reaching out to Microsoft Q&A.

1. May I know which specific app you're trying to set as the default, and for which file type or protocol (e.g., PDF, browser, photo viewer, etc...)?
2. May I please know if you've signed in with your Personal or local account on your device?

Run a system file checker utility scan to fix corrupted system files.

Open Command Prompt as Administrator.

Execute the command below and click Enter.

sfc /scannow

Restart your computer.

If your default app settings are still not saved, the problem could be with your user profile. If the system file checker scan did not resolve the issue, we can try creating a new local user profile next and check if you're experiencing the same problem there.

If you're having issues with a specific application, you can try reinstalling the particular app and see if it works. Also, check if you have all the updates installed.

Let me know how it goes.