Got some similar issues on group chain spawn.
Tried with ME trigger check as on start time delayed GROUP ALIVE (A) with GROUP ACTIVATE (B) action, and the result is that action is being done at mission start(!) regardless if late activation is set on GROUP ACTIVATE (B) units.
Tried with script trigger.action.activateGroup('B') and there comes the error - method activate as nil value.
Something is missing or wrong definitely but the triggers in ME are suspicious.
Using MIST as framework.
This script works though Group.activate(Group.getByName(groupName))
jackmckay's post in trigger.action.activateGroup(group), ME, error(s). was marked as the answer