You can select a unit that is part of a group, without removing it from the group. That's when these oddities (of orders buttons not available) occur. When you select a group, you have an orders button with iirc 3 arrows diverging, to 'disband' the group into individual units. When you select a single unit while it is still in a group, there is a 'reciprocal' orders button with the arrows converging. I don't know what it does, except that clicking it makes it go away, and you have to do this before including the unit in a merge (and probably also before splitting it). Mysteriously missing automation buttons follow the same pattern.