I don't know what is causing it. Do you have more than one account? If so, you need to turn off the setting for all accounts individually.

You said this just started happening. Brian spoke of possible corruption of the program and that could be. You may want to uninstall and install it again as he suggests. Something detrimental evidently happened since you didn't have this behavior before.

Even if you solve the problem, you may still want to consider the following:
One thing I encourage people to do is use Thunderbird portable. You can keep your installed version and try the portable version so you will have no delay or interruption in receiving e-mail.

The reasons I say that people should use the portable version are that you can move the program to any machine you want and have all your e-mail and settings, except that I don't know if you will have message rules transferred.
You can run it from something like a thumb drive if you are using a machine that isn't yours and that you don't have the program on. And you can keep a backup copy so that if the program becomes corrupted, you can simply delete it and copy the backup copy to the location where you usually have the program. To keep the backup copy current, you can delete the backup when you want and copy the one you are using to that location again. This will preserve your e-mail and any changes you have made to the program, such as settings changes and probably add-ons as well.

You won't have to trouble shoot problems anymore that arise from corruption of the program. You can just replace it by deleting it and copying the backup copy to the location you run the program from.
Although I use the Brave browser for most of my browsing, I use Firefox portable off and on and I use the portable version for the same reasons.

I use portable versions of programs where they either do everything the installed version does or where the portable version does the things I want it to do.