I was forced to fly twice during the brief period in which Spain/UK travel didn't imply quarantine. In both occasions I was assigned the middle seat of an empty half-row. In one of these cases the whole row was empty. The planes were around 1/3 occupation at best.
I haven't read how Ryanair claims to assign seats but it's clear that within the randomness they have rules. They save window/aisle seats for those paying. When you are assigning middle seats first, obviously couples will end separated. This happened also to me in one of these flights, I was in the front (row 6 IIRC) and my travel partner was in a twenty-odd seat.
I find it ridiculous; given that paying for a seat gives you weeks to pick the seat in advance, in the last 48h the non-paying pax can check in it would be minimum politeness to keep people together. But I guess last minute travelers paying for a seat must be kept happy (or less disgruntled).
Once you're made aware of how your system works, incompetence/shortsightedness/cheapness/whatever does turn into malice. The result is a cold war between you and your customers, or should I say your captive clientele. I would never fly Ryanair if I had an alternative for my means. Splitting couples out of greediness has some offensive quality that crosses a line. They should at least factor in the expected plane occupation, particularly in covid times, to recover some goodwill.
Likewise they could be frank about it and it would be less insulting. I can understand (particularly as an often solo traveler) they saying "we save window/aisle for paying customers" but they have to say "you'll be assigned a random seat, probably a middle seat" as if it were a magical occurrence.
Sorry for the rant. I work in optimization and all this boggles my mind. But then, they have the numbers.