@021 Without actuallylistening to the song myself, I would say that this ♭III chord is kind of justification in and of itself for the F♯ and C♯.

You're correct that the two notes are not diatonic, so it makes sense that they might sound a bit out of place in the sequencer.

That chord progression looks (superficially, though) like the kind of progression that might utilise some chords borrowed from another tonality, like the ♭III chord (some other common examples include ♭VII, ♭VI, and possibly ♭II).

Actually, lots of songs do use the notes from a borrowed chord in their melodies, so don't think that that's forbidden or anything. It doesn't seem that your ♭III chord is really doing much functionally apart from sounding cool.

But of course, I haven't heard the song, so take this with a grain of salt (If it's a jazz song, for example, this could be functioning as a subV/ii, and there are plenty of other possibilities that vary based on the context).

Thank you for the insights @user45266 it is very interesting ! This kind of elements really intrigue me, because, not that I'm thinking it's forbidden, I would have thought that it would have sounded very much more unsettling that it does in this song. It's making me want to learn more about modes, when to use or not use to create what feelings.