Question what the use of this trick is when you want to play "release as a vassal" and fully inheriting it.

In a planned "release as vassal"-campaign what is the point of seizing a province of the releaseable vassal and make it capital and provoking seperatists there?

and then also attack it instanly again for -5stab

e.g. "Forming Rûm as the Ottomans is INSANE - EU4 1.36" (by ThePlaymaker) in the middle of video he does the trick

For comparison in my run I did give big part of my provinces to my vassal then full annex and released him and also got all provinces he had.