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.
- Create a scoreboard objective and set a score for a fake player
- Type or copy the following and tab-complete the name
→ It replaced #fakePl with the missing ayer
Note: The amount of characters you had previously typed is afterwards highlighted.