Twitter mentions not showing up for a specific user


Hello. So, I’m using the twitter API in a simple Node.JS script that listens to mentions and replies something. It’s using an account made for the occasion, and it’s working pretty well.
I’m encountering a problem though, with a specific user (and probably with another one, but I’m sure about this one). It can’t see the tweets where he’s mentioning my account; either on Twitter Web or using the user streaming API. It just doesn’t show. Also, I’m not blocking that user, and he hasn’t blocked me, so I don’t see what could be the issue, really.
Is that a known problem, or what? My account is @Rand0matic and the user is @jean_francois, if that’s needed.