It could be an issue with SH herself as someone has already pointed out. In my most recent save where I romanced Astarion, the other companions just backed off after I chose him in the tiefling party, high approval be damned. Then fast forward to Act 3 where I get propositioned by Halsin, Mizora, and Empy on three consecutive nights XD
In any case, I do think they need to add more options for the PC to react to the advances. It wouldn't be a lot of work to add a couple of lines for every companion.
