The bug

The command /scoreboard players enable tab-completes and allows non-player entities.

Note: This could be intended since it is not causing any problems and internally possibly uses the same format as for players.

How to reproduce

1. Create a trigger objective

   /scoreboard objectives add testTrigger trigger
   

2. Try enabling it for non-player entities

   /scoreboard players enable @e[type=!player] testTrigger
   

   → It enabled the trigger for the entities