Investigation of this issue is complicated by the fact that the editor test widgets are wrapped by multicall.MulticallCreator.  It intercepts bind and event method calls (other than event_generate) for user pseudoevents.  It keeps its own map of pseudoevent names to handler and key sequences instead of passing the information on to tk.

The custom entry for "<<autocomplete>>" is the same,
 [<bound method AutoComplete.autocomplete_event of <idlelib.autocomplete.AutoComplete object at 0x00000209DA4FBD20>>, [(0, 0, 'Tab')]]
both before deactivate_... and after activate_... .

I determined this by adding calls like the following
          print('ei5 ', instance.text.event_info('<<autocomplete>>'))
and adding the following to the event_info(virtual) override.
          print('mei', virtual, self.__eventinfo.get(virtual))

I will next look at the custom binding data that should map keys to event handlers.