Well---your ex messed up. If you were legally married at the end of 2020---and you say your divorce happened in 2021----then your filing choices for 2020 were to file married filing separately or married filing jointly. But a joint return requires that you both agree to that---he did not have the right to file a joint return without your agreement to it. If you already filed MFS----did you e-file? Was the return accepted? If so, then it would not have been possible for your ex to e-file a joint return with your SSN on it---it would be rejected. If he mailed it, then you are both going to get letters from the IRS someday and he will have some explaining to do.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
