Hi y not,
“Is it the case that ‘once a tihetuka, always a tihetuka’ ?
That is, once a tihetuka no future bhava will be either dvihetuka or ahetuka?
(Parallel with the case that when one attains magga phala there is no going back?)”
No, because like we know brahmas have tihetuka birth (by definition), but they are still subjected to rebirth in the Apayas if haven’t attained magga phala yet.
On the way to Stream Entry.