Actually, it all depends on how many panties the script drops - it’s not guaranteed that 90 will spawn. In any case, the maximum reward is given if you collect 81.
If you’re talking about Season 1, there shouldn’t be any bugs there. The game has been completed many times by me and other players. You should follow the events marked with the heart icon, increase your relationships with the girls, and explore different locations to progress the story. It’s also important to buy transportation.
In Season 2, there are hints for every stage of each quest — both for the girls’ quests and the main storyline.
Due to negative feedback from the community, this route was closed in version 0.19 and will be reworked. You can unlock the scenes by downloading version 0.18, or wait until the route is reworked in future updates.
As I understand it, you’ve already found some of them by spending time with the characters. You can find the others in a similar way, but you’ll need to spend time with several of them in a row, and then they will appear.
I won’t say it directly, since the update has just been released. Maybe if you don’t figure it out by then, I’ll post it on Discord a bit later. But I’ll give you a hint: both the panty search and the problem with Yu can be helped by spending time with the residents of the house :)
