yevamos 87b not being able to eat truma, d'robonon??

the mishna says that if a woman remarries and then her husband shows up then she is pasul for trumah and rashi explains that its m'drobonon.
my question is that in 35a in the case of 2 brothers and 2 sisters that exchanged their wife’s by mistake, there the gemara says that they are pasul to kohanim, rashi and tosfos debate the source for that, according to rashi its from the pasuk of having relations with a "zar" like amoni and the like (68a), problem is then she should be pasul also to eat truma forever m'doraysa. bottom line according to rashi 35a it would seem that in our mishna also it should make her not be able to eat truma m'doraysa???