User cannot log in to domain joined mac without me

Hi,

I have a Windows Domain Joined Macbook Air, running Catalina and fully up to date.

I set up the mac for him, and logged in with my domain administrator account.

I have made the user an administrator and he logs in with his domain credentials.

Then one day a few months ago.. after a reboot, he could no longer log in.

It wasn't his password, we changed it, and I tried to login for him as well and nothing.

So I thought I'll log in as me to check it out.

Couldn't find anything, so logged out.

Thought to try one more time to log in as him.. and it worked...

So fast forward, basically, every time he reboots his mac he can never log in, until I've logged in and out, then he can log in fine.

I have 8 Macs in the office, and this is the first and only mac I've ever seen do this.

Does anyone have any suggestions with fixing this.. without reinstalling Mac OS, or his user account?

Thanks,

Tim