I've had two yellow egg creechers in a row who really struggle to gain sun affinity, even with sun charms.
https://dopcom.net/creechers/p/room/qzq1ACT9
https://dopcom.net/creechers/p/released_pet/qzq1ACT9ACV9
The released creecher had equal amounts (three each) of wood and sun charms. The creecher still in the room has three wood charms and five sun charms. Regardless, after the first day of equipping the charms, where they gained similar amounts of sun and wood affinity, they seem to only gain wood affinity; their sun affinity just keeps shrinking (it's also possible that they're still gaining sun affinity, but at a reduced rate compared to wood affinity).

I'm not really sure whether this is a bug or I'm somehow doing something wrong; I don't have any other pets with charms of multiple types to check them against.

Because the wood charms were equipped first, they're going to trigger first every rollover. If the pet doesn't have enough available power, that could result in gaining so much wood affinity that there's no room to gain any sun affinity. While the pets were growing, did the affinity meter look full most of the time, or was there empty space in it?

I don't remember how the released creecher's bar looked as it was growing, but the one currently growing usually has some empty space in the affinity meter each day; it's usually somewhere on the order of one or two bars, though (though it may be filling up right after rollover and before it's fed). I'll try alternating charms on my next mixed-affinity pet haha.

Thanks for the advice; I'll keep an eye out after rollover!