Mark phone number as 'cannot receive iMessages'
See if you can opt that SMS number out of iMessage entirely at https://selfsolve.apple.com/deregister-imessage as explained in more detail at How do I disable iMessage for a contact who no longer has an iPhone

If that doesn't work, contacting AppleCare to enlist the support of an engineer might be needed. In the short term, you can disable iMessage entirely and SMS functionality should be restored for all SMS eligible costacts, but you lose iMessage for everything else as well.