Players switching roles when making formation changes

I've set up a very offensive formation in the Tactics screen (outside of a match), with all the players in their most suited/most comfortable positions and roles.

However, when I created a new, more defensive, formation (with the aim to train both at once), around half of my players are automatically assigned an uncomfortable position/role. Worse still, this somehow shuffles around the first formation I set up!

Anytime I make changes to one formation, it seems to have some impact on the other. Is there a way to set up two (or more) formations, such that all players are in their most comfortable role?