Yeah I am trying to figure this out. I have not been able to reproduce it. It should not be possible; I think the UI state is getting out of sync with the true puzzle state. It looks like you tried VIALS - do you know if you interacting pretty quickly? Using the keyboard or not? Another person reported weirdness with tiles that were supposed to be getting locked in place instead being replaceable. I wonder if the final row was merrily animating its lock into place sequence, then you typed VIALS, and it permitted those to moved, falsely locking them into place in the other word?

I was using the mouse to drag around letters, not the keyboard. It was my first time playing and I was still wrapping my head around basic game concepts, so it's unlikely that I was interacting quickly.