I think you're on an old version. The issue you have is that the squads don't revive properly after being defeated. This causes them to spawn into battle "invisible" and thus create tiles that other squads can't pass through. In the most recent version, 20.07.19, the problem should be fixed and you squads revive after a battle.
(And btw, you should have 6 squad leaders in total, not 5).