Of course its okay to forget.
You have to understand that WaniKani is just a means to an end. Its designed to allow you to efficiently recognise Kanji and associated Vocabulary in the long run and many people lose sight of this.
The end game here is to be able to read novels, websites, manga and what not. You don’t truly ‘know’ a Kanji’s meaning or reading until youve seen it in its native script somewhere in a book or something, and can read it without even thinking about its meaning. This is irrespective of whether you’ve burned the item or not. Granted, the fact that you’ve burned it increases the chances that you come across and think ‘Oh i know this!’
A good example for me was 何. Sure, WaniKani taught me its meaning, and the various ways to read it. But it wasn’t until i kept crossing it in written text that I truly ‘understood’ it. Now, I dont even think about its meaning or reading when I come across it because I unconsciously understand it (until i come across its use in a way that I havent seen before). Too bad this can’t be said about most of the other Kanji yet -__-
Just like almost every other activity it goes through a cycle. 1. you dont know what you dont know. 2.you realise how much you dont know. 3. you know you can do it, but it takes effort and 4. You dont have to think about doing it. WaniKani takes you to step 3. Step 4 you have to achieve yourself outside of WaniKani.
Sorry for the wall of text! Id be interested to hear your thoughts.