I think this problem solve if you understand two rule from tipika.
1 the ordination use at least 5 bhiku and 5 bhikuni.
2 5 bhiku and 5 bhikuni will came from same school.
thai sangha analyse srilangka bhikuni is mahayan like their origin
taiwaneese bhikuni.
please understand, thai sangha comunity for protected the rule from tipika
"We don't chang any rule" is core of theravadha.
was this laid out by the Buddha?
whats the rationale?