I added my daughter's school account in Teams on my phone so I could submit her homework.

Later, I filled out my taxes in Excel and saved them.

It had uploaded them to my school's default OneDrive shared folder. It never asked me if I wanted to use that account as my default, and never told me it had changed accounts. It took me 10 minutes of non-sensical "file is locked" messages before I could delete my private data from my school's drive.

Some apps such as Microsoft Authenticator won't even let me remove the account.