Self-explanatory, tab completion still works perfectly even when it should start in the second character.

https://www.spigotmc.org/wiki/spigot-configuration/

tab-complete
Above #1368:
Default: 0
Type: Integer
Description: You can now specify how many letters of the command must be typed before it will be tab completed this will help deter people from just spamming round all the commands to see if there is one incorrectly set up.
0 will tab-complete all commands
-1 will disable tab completely
1 will mean you have to type the first letter
2 will mean you have to type the second letter... etc...