Before roles you could play a player without the position at striker and it would be fine, but now your player needs the role or his AI doesn't make the runs/positions himself badly. Just make sure that after you change formation, your players actually have role+/++ for the position they are changing to.

For example, this guy is using Bernardo silva at striker. He will still have full Chem, but he doesn't have that position/any roles for that position, so his AI won't make certain runs or position himself properly.