Huh? How so? The dad doesn't know Taiki like we the readers do. All he knows is she's living with the boy she's dating. Its a pretty responsible and realistic response.
Taiki just pulled a 200iq move.
not really no, weve been told and shown that Chinatsu's dad is stoic to the point of barely allowing her to stay (which took convincing from her and her mom), we even have been hinted at Chinatsu being some sort of "afraid" of him (although i think its more that he feels unaprochable to her)
so after hearing that her dad would only be in japan for a month or so, their most sensible move would be to just not mention their relationship (im pretty sure they havent told Taiki's mom either btw)
you can write this off as Chinatsu taking the iniative, being sincere because she knows she isnt doing anything inapropiate (which is half true, we know she has a big desire of touching Taiki, teenagers being hormonal makes this a big risk even if the series doesnt touch on it), etc
the correct way for this chapter would have been something like: first, Taiki's mom knows about the relationship, then when the father is visiting, so is the mom, Taiki's mom being the more talkative let it slips that theyre in a relationship, Chinatsu's father gets furious, and before storming out tells Chinatsu to pack her stuff... then we can finish in the same way as this chapter did
overall the chapter is weak, but it was 100% to set up Taiki staying with the father for him to get to know him and be ok with the relationship, weak but not the worst the series as put out for sure