I was able to reproduce it; if you do the test piloting of the Genea and have 5 Helena love, "Date With Helena" shows up on Treemin. If you go on the date with her, afterwards you can access the conversation where she asks why you didn't let her go, even though you haven't done that stage of the quest yet. And doing that conversation at that point means you get stuck at 6 Helena love with no way to get more, so you get shut down on the very next conversation, before you even get to the step which requires the "Family romancer" achievement.
Probably it's this line from the Treemin hub:
"Date with Helena" if not is_done("helena_date") and hel_best >= 5 and main_story >= 3 and not hel_dec:
I think main_story >= 3 is too soon, you probably meant 4 or 5?
Anyways, the problem remains that even if you wait, do the main story first, let her go, and then go talk to her, she still doesn't think that you let her go. helena_letgo doesn't have a default value set in variables.rpy and I don't see it anywhere else, either.