The bug

The /scoreboard players operation command tab completes fake players for the first but not the second name argument.
Additionally the syntax help calls it "selector" instead of "name":

/scoreboard players operation <targetName> <targetObjective> <operation> <selector> <objective>

How to reproduce

1. Add an objective

   /scoreboard objectives add test dummy
   

2. Set for the fake player #fakePlayer a score

   /scoreboard players set #fakePlayer test 1
   

3. Enter the following command and try to tab complete the fake player for both name arguments

   /scoreboard players operation 
   

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.command.server.CommandScoreboard.getTabCompletions(MinecraftServer, ICommandSender, String[], BlockPos) uses only online players for the second name argument.