If you set a collision group not to collide with the default collision group, the objects in said collision group acts as if they are locked, and cannot be selected.
demonstration:
replication:
Create a part
Create a new collision group, set it to not collide with Default.
Set that part to that collision group.
You are no longer able to select that part by clicking it. you can still select with through explorer, or dragging a selection box around it.
I’ve seen no documentation about this, so I’m fairly certain this is unintended behavior.