(bug since 1996)
In key control mode, if you're in a group and activate panic mode (left click +right click), all agents move to their positions as expected, but are then unable to follow or be followed by anybody from the same group, regardless of regrouping. Combining two groups affected by the bug doesn't do anything special.
If you select an agent that wasn't affected however, regrouping fixes agents in the group. Selecting an affected agent and grouping a non-affected agent also fixes the current agent, and selecting the non-affected agent fixes all agents in the group. Issuing a movement command (left click) for any affected agent or group also returns them to normal.
Very easy to reproduce. Go into any level, group all agents, enter key control mode and activate panic mode: the agents stop following other agents from the same group.