Please wake me up when they turn this back into a rom-com. When did we last get some romantic heat? Chapters 88-90?
Get something going in Gojou's head maybe. Build a romantic triangle. Something! Slow burn is one thing, but this seems like they dunked the sparklers in the water bucket.
That's actually a natural reaction after all the development we had, we all know that Marin, is actually a bit romantically awkward, she saw what happened to Wakana and it befuddled her, she can't wrap why he had such a sad face when she was having the most fun of her life, she was afraid of actually confronting Wakana, Again.
This is the same hesitation she'd been having when she said to herself that she want to confess.
In other gal manga, usually this is where the girl actually being a tad insensitive and directly confront the Mc, but no, we still got this lul instead, because she can't find the good moment to actually bring that topic up (which is normal with both kids' guardian on site) she choose to be quiet in this visit.
It's actually a normal choice for people to wait it out.
In conclusion : she is afraid that she'd mistaken all his actions as his kindness and that actually its torturing himself while being with her all along.
This is a misunderstanding on her part and actually a good character design, showing a gain that Marin is not simply a perfect "gal girl" made for the story. She has faults too