I think she wasnt his lover in the first place, in the past he simply didnt have anyone he loved, so pretending to be her lover wasnt that big of a deal. Most likely it was some kind of plan, theres no way he was actually in love with her. He either needed someone just for show or those were just rumors that he allowed to go around since he didnt give a shit
The trope here makes sense though, in the past life and current, Ramona appears a someone part of Perez's group (from the academy), so whatever is in the academy didn't really change since Tia isn't involved there. It makes sense if Tia assumed the same, as they really are engaged in the past life she's seen.
I mean to be fair in her past life, she wasnt even directly involved with the second prince. Its understandable to assume since she and the second prince also were both in the academy together and became lovers from there. (Assuming) so yeah unless she gets a verbal confirmation thats all we get
here comes that dumbass trope of “oh they used to be together in my past life so they probably will be in this life too” disregarding the fact that she’s changed so many things including her fathers death that this could be changed too