Senior Feline Neutered/spayed Bonded Pair (10, 15 Years Old) Engaging In Mating-like Behavior?

hi vets! i posted about this in one of the animal behavior groups on FB, where trainers and behaviorists respond, but didn't receive any feedback. this is completely non-urgent and only a matter of curiosity, so i get it, but...i remain curious!

i did ask my vet about this, but they kind of just shrugged and said it's just one of those things cats do sometimes. as a lifelong multi-cat owner, i've never seen it. it's not causing a problem, i am just really curious about it and googling hasn't yielded much.

we have 2 males and 4 females of varying ages, all fixed - the two sexpots in question are a 15 year-old male and a 10 year-old female. they were never, at any point, around one another in an unfixed state. if it's helpful to know, the male was neutered at 8 weeks, and the female did reach sexual maturity before we rescued her and had her spayed immediately - older than a year, less than 2 years old, our vet's best guess when she had her first exam. never had a litter.

about 5-6 years ago, they started this weird ritual where the female yells (she's a vocal tortie, so not entirely abnormal for her) in a particular way, almost like a cat in heat yowling. it's repetitive and loud. when she does this, this male - without exception - runs to her, trills at her with a little weird meow, and, well...they do a really clumsy and bizarre version of what i guess is cat dry humping. like, they "assume the position," he attempts to scruff her with his teeth, but obviously...nothing is happening. probably 75% of the time, the female isn't even on her stomach, she could be on her back or on her side, it's just absolutely goofy to see but they take it VERY seriously. it's like clockwork and always unfolds in exactly the same way, just at varying times of day and with varying daily frequency. i work from home, so i have many opportunities to observe this whole spectacle.

after maybe 2 minutes at most, one of them gets irritated with the other and they separate and then "make up" by grooming each other. this entire chain of events happens at least once a day but it could happen as many as 4-5x. no particular time of day, they could rise with the dawn or set with the sun.

i find it so odd because a) it's unique to this pair, whom i would absolutely describe as uniquely bonded, b) i've never seen it before in my life with cats and c) it started 4-5 years into them living together.

if it's of interest, as i said, the tortie is VERY vocal. she's one of those that "catches" toys and other items and brings them screaming into the room, sometimes seeming to want acknowledgement from us humans, sometimes from the other cats. the sound she makes then is SIMILAR to this "i'm a lil delinquent" meowing, but different, and the male cat does NOT react to the "look what i caught" yowling in the same way. he may check out what she brought, but it's not at all with the same urgency or sense of purpose. when she's doing "the other thing," he could be in a dead sleep and he'll get up and go find her. it sounds different to me, and apparently it sounds different to him, too, but i'm not sure that a non-crazy-cat-lady would pick up on the difference without hearing both sounds over and over and over again for years. the other cats in the household have no reaction to the "sexy" meowing and show no curiosity or interest in the resulting behaviors.

as i said, outside of this weird behavior, they are close and affectionate with one another. they never fight or display other dominance-related behaviors. i know it's said there is no such thing as an "alpha cat," but if there was, the female would be ours, and the male is her righthand man whom she never "corrects" or gets snippy with. they like to be within paw's reach of each other and spend a lot of time together. the male cat is easygoing and never involved in even minor conflict, although he is nosy and wants to go see 110% of whatever is going on if someone hisses, knocks something over, zooms through a room, etc.

i'm sure it is not relevant, but the female is totally healthy. the male has an unidentified tumor/growth in his sinus/nasolacrimal area (present for approx. 1.75 years, many vet visits) but is otherwise healthy with clean labs and no systemic signs of illness at this time.

again, not important, it's not a problem for them (as far as we can tell) or for us, i am just really curious about if this is a normal thing, or if i've got some weird behavioral outliers on my hands. when i've googled, i've seen that neutered cats sometimes retain sexual behaviors, but they seem to be referring to the months immediately post-neuter...not a senior cat who has been neutered for 15 years now and did not reach sexual maturity prior to neuter.

ETA: i realized i didn't clarify this - when the female is yowling/crying in her "special" way, she's extremely persistent and on the RARE time he DOESN'T immediately jump up and go to her, she'll move closer and closer and get louder and louder until he does. also, the male cat has never humped objects, other cats, etc. - not once. it is solely directed to this one female, upon, seemingly, her request.