Isn’t it weird that one of the acceptable meanings for 下がる is “to get lower”, but “to get higher” isn’t one of the acceptable meanings for 上がる ? Unless there’s some nuance of the words here that I’m not getting. Are these words not equivalent in that way?
Yes, in context it could mean to get higher. WaniKani doesn’t list all possible translations for a word.
WK defines 上がる as to rise, to go up, to be raised. It could also be translated as to ascend, to come up, to increase (i.e. prices), to emerge (i.e. from a bathtub), among others.