Neutered male appears to be mounting other cat

TLDR: male cat (5) was neutered at 6 months but appears to be mounting (spayed) female cat (2-3). Is this even possible?
Edit: yeah so after some observation and discussion with the roommate, they seem to just be playing rough. Oliver is. A bit socially and mentally delayed while Cori is very young so they’re both very kitten-like sometimes. I think with time, they will chill out a bit and I won’t have to step in when they’re playing too rough. I do try to let them work it out on their own but Oliver is massive and actually poses a threat to Cori, not that either of them realize that lol.

Im at a total loss at what to do. I adopted a cat about three months ago. My roommate has a cat that I am very bonded to and he’s been having trouble adjusting. My cat is Cori (2-3years, spayed in June). Roommates cat is Oliver (5, neutered at 6months). Oliver is also 2-3x Cori’s size. They mostly get along except when Oliver is in a mood. He’s started latching onto her neck. He never bites hard enough to break skin but it’s distressing for Cori. We’re likely going to do a proper reintroduction when we have the time for it but right now we’re trying to redirect which seems to be working somewhat. In the last couple days, it’s almost like he’s been mounting her from his positioning and i saw him making sin biscuit like motions though it devolved into bunny kicks when he flopped over so it seems like maybe just adjusting? Is it even possible for a neutered male who hasn’t been around adult cats except for about a year as a kitten to even have the inclination to do that?? Is it just an unfortunate angle that he’s grabbing her?? I just have no idea how to even resolve it if that’s what he’s doing.
Cori was a shelter cat and has been through so much. I just want her to be happy and healthy. It doesn’t seem to bother her too much except when it happen of course.
I’ve looked into cat bullying and from what i can tell, none of the regular triggers apply. We have separate feeders, multiple water bowls and litter boxes, and fights never seem to occur in the proximity of any resources. The cats each have their own spaces that the other doesn’t go so they can have alone time plus I make sure Oliver still gets plenty of attention from me and my roommate.