Like today I came across 壌 (lot) and I knew how to compose it but I couldn't quite remember the "grass skirt" primitive so I gave it the "Soon" in Anki. Then a few frames later I come to 嬢 (lass) and write it without hesitation. Is the proper thing to do to fail it as well? Since had I not failed 'lot' then I wouldn't have seen grass skirt correctly written and it wouldn't have been fresh in my mind.

I believe there was a topic about this not too long ago, and if I remember correctly, the consensus was that no, you should not have to fail it. If you fail "lot" and review it, then you will be simultaneously reviewing "grass skirt", which was the only missing piece of information; given that, you were able to figure out "lass". If you had failed both of them, you would just be reviewing "grass skirt" twice, which would be redundant.

I hope that made sense =/

Yeah, that makes sense. I used to fail the ones with similar primitives but I recently stopped and was curious as to whether or not it was the right choice.

I used to always wonder the same thing. Currently, I do not fail cards like anymore and it hasn't hurt me at all.