For what it’s worth, I managed to replicate this today.
For example, you want to sub off your right back, move your left back to right back, and the new player coming on slots in at left back.
If you make the substitution, then before it’s taken place use the tactics screen to swap your left back with the player coming on, both players come off. If, however, you swap your left back with the player coming off, they just switch positions on the pitch.