To reproduce, create a new trigger, assign yourself a score for it and trigger it once so it no longer is active:
/scoreboard objectives add testTrigger trigger
/scoreboard players set @p testTrigger 1
/trigger testTrigger set 1Then try to tab-complete /trigger testTrig and see how it fills in "testTrigger" even though you can't trigger it.
I would not mind if this is WAI. (Autocompletion in many tools is mostly 'schema based' (where 'schema' is a set of types/properties that rarely change) as opposed to 'completely dynamic' (based on the moment-to-moment state of the system). The types/set of objectives rarely change, but the 'triggered' state is likely to change many times at runtime during the normal course of game logic in a command-block contraption.)