I had an eyebrow raised at me (I'll put it no stronger) on one bwtcamp for using "jyst".
I wanted to reply (in Welsh of course): "but in my head, I'm spelling it jyst, no matter if it sounds like just to you". (But my spontaneous Welsh wasn't good enough to express that idea... )
I knew I had heard it loads of times within Welsh sentences on Radio Cymru, but I was even less well able to express that idea (at least not spontaneously) in Welsh.
But chwarae teg, perhaps we learners should sometimes stop and try to make the extra effort not to take the easy way out and use a word that sounds like English, even if it's also often used by "proper" Welsh speakers.