Password to edit, but password no longer on the spreadsheet

I've got a spreadsheet that previously required a password to edit. I've since removed that password requirement and it opened okay for a few weeks; however it's now reverted to asking for a password to edit again.

I've checked into the field where the password was (is) held and it is blank [Tools > General Options] and yet it still asks for this info.

If I change it so that it does require a password to edit, then it does require the new password that I add. And if I then subsequently remove that password, then Excel still requires me to enter the new password that I have just added.

Any thoughts as to what's going on? I've asked an AI and it had a few suggestions, all of which I've tried, but I'm still at the same place.