The bug

When you try to tab-complete an incomplete fake player name starting with a # the missing part replaces what you have already typed instead of being appended. It looks like # in the middle of the name does not cause this.

How to reproduce

1. Create a scoreboard objective and set a score for a fake player

   /scoreboard objectives add test dummy
   /scoreboard players set #fakePlayer test 1
   

2. Type or copy the following and tab-complete the name

   /scoreboard players set #fakePl
   

   → It replaced #fakePl with the missing ayer

Note: The amount of characters you had previously typed is afterwards highlighted.