Ok, just to let you know, the tentacle quest isn't set for Lace's bad route. I got the quest even before I captured her and I was still able to build it. As for the bug, I'm not to sure about that...
Well, aside from that, I see no other reason to why I can't complete the tentacle spores quest. The guard has disappeared and I can't interact with the tentacles in the Hard Dungeon. I just think that maybe the Lace Reset affects it since her bad route uses the tentacles and hints at acquiring them. Maybe the reset works in a way where the route is completed, as in tentacle spores planted, before Lace gets actually rewinded. I don't really know at this point. I'll continue looking around; I could just be an idiot but it sure has taken me some time. Thank you for your insight!