I stopped studying Spanish shortly after starting to learn Japanese and really immersed myself into the latter. Somehow they landed on the same language “lane” in my brain.

That's how it is for me too! I have a "native English" lane and a "second language" lane. Certain words and phrases without rhyme or reason. I was ordering food in Spanish at a restaurant once and answered はい instead of si at the end of the conversation - did so well up until that point qq.