Wait hear me out here, in season 5 when Dexter tranqs Boyd Fowler and in response Boyd hits him with a tranq dart (which Boyd implied would be used on the gator if it was alive) which means the tranquilizer would probably be one of
these things (xylazine, detomidine, romifidine, acepromazine (a tranquilizer) and butorphanol (a synthetic narcotic)). All of which would have made Dexter very sick if not just outright killed him but he was completely fine afterwards.
Meaning that at the very least Dexter could keep his poison resistance just not with the M99 justification anymore.