I have recently asked this question and a Japanese guy has answered:
逃がす is letting it go. 逃す is missing it.
例
釣った魚を逃がす let the fish go.
チャンスを逃す miss an opportunity.
In the Wanikani meaning explanation, it is stated that they have the same meaning, i.e. to let go.
See WaniKani / Vocabulary / 逃がす
But when I look at the example given for 逃す, the meaning used in the sentence is "to miss”, which cannot be a synonim for “to let go”. See WaniKani / Vocabulary / 逃す
Basically 逃がす is to actuallly release something from captivity, while 逃す is to allow something to get away from you. WaniKani does make that difference reasonably clear in the meaning explanation sections, but doesn’t really make a point of hammering it in.
You might want to move the category of this post and mark one of the comments as a solution - so others know your problem is solved, and it’s not actually a bug in WK… unless you think you need to tag JenK or something.