Other than that the Cauldron is pro eugenics, I also like to think that the mating bonds aren't about who's compatible with the soul or even about offspring, but has to do with the magic of people themselves. Like for Rhysand's mom and dad, maybe her magic was calling out for help (I also like the idea that all fae are magical in some way even if it doesn't manifest) and his magic instinctively responded to hers. The mating bond could literally just be magic merging than any sort of divine interference. Same with Feyre, the mating bond only clicked with Rhys until she became Fae.
There could even be an element of unconscious choice. Rhysand's mother didn't necessarily want to have a mate, she wanted to be saved. The same could honestly be the same with Feyre. And it could also be why the mating bond doesn't click for her until she actively wants it.
Tbh it would be interesting if the Fae have a misconception of what mating bonds are at their core anyway, given that so far, everytime a mating bond clicks, it's usually one sided.
Rhysand's mom to his father
Rhys to Feyre
Cassian to Nesta
Lucien to Elain
Someone always receives a bond and the other has to accept it. This could honestly be just someone else's magic reaching out and then being accepted by the other person.
Edit: Kallias and Vivienne could be a case of mutual mates (but didn't sjm also say that one of them knew first??). Same with Thesan and his lover, but we literally know nothing about them.
Another edit: could also be why there are no human/fae mates