Hello!
If we run a txt lookup on your subdomain its SPF record for mail auth shows up
Can you please temporarily disable your SPF / DKIM txt records and try again?
Its because in a DNS zone, TXT records get "concatenated" into a single TXT entry maybe your facebook validation record has found itself at the end of file hence why facebook could not recognize it.
You may also expect a delay for validation to occur: up to 24 hours usually
Once validated successfully you could recreate them easily in cPanel / email / email delivrability
I hope this was helpful but if it was not then don't hesitate let us know.