The bug

/scoreboard only accepts # when it's the very first character of a fake player name. Using # elsewhere in the name requires quotes, even though there is no ambiguity.

How to reproduce

/scoreboard objectives add test dummy

/scoreboard players set #test test 1
/scoreboard players set test# test 2
/scoreboard players set te#st test 3

Expected result
All three "set" commands succeed

Observed result
Only the first one succeeds, the others error

Notes
The only character that should be unparseable without quotes is the space character. So this command should work as well:

/scoreboard players set ~!@#$%^&á12345 test 1

Please ensure that this is the case when fixing this issue