Sh triggering first romance scene before the party is not a bug I beleive. It is triggered from a conversation option that you missed an opurtunity to know eachoter a little better (something along those lines), also since this is a longrest scene it means you have either missed some triggers for other longrest scenes or have been generous with your longrests. Also means that you have hight enough aproval rating to trigger it (as SH is a tad hard to get compared to some other romace options). Never experienced that I can't give her the flower, but then again I always chose the line to talk about it in act 1. Without knowing if you went the light or dark route with SH, it is hard to tell what triggers you missed to not get the scene in act 3. For the dark route I think you have to do the House of grief part first (not 100% sure though, as I have only done that once). For the light route it can take some longrests as there are some other scenes "in que" before that one.
