Local admin account demoted, PC dropped off domain

[Sonic3]

Having a rather annoying issue with Windows 10 PCs randomly dropping off the domain, which wouldn't be too bad if I could just log on as the local admin account and get the PC to rejoin the domain. However, I'm finding that the local admin account has been demoted to a standard user which means it doesn't have the necessary rights to join the PC to the domain again. Very frustrating. Any ideas why this might be happening?