So, I'm not entirely sure how to describe this issue.
I recently re-launched my SMP server and wanted to add some commands to it and chose to use vanilla BDS as opposed to modified software (for feature consistency). WE are using the chat detection system in the Gametest framework for this.
We tested it in a multiplayer setting in a hosted world and it worked flawlessly, however, when put onto the server, even with only 2-3 players, it'll randomly not detect some player's messages, but will detect others. The consistency of the detection is entirely random, one moment it works for me, for example, and then when I relog it stops working, when someone else relogs, it'll suddenly work for them.
I have attached the addon in question for further testing. 

How to reproduce:
Apply the behavior pack provided in the report in a singleplayer world
All commands use "." instead of "/"
Attempt to use ".spawn", you should be teleported to "0, 116, 0" (location of my server's spawn)
Have any number of other players join
They should all be able to properly use ".spawn" as well.
Apply the behavior pack in BDS
Have the same players join as before
Some players will be able to use ".spawn" to teleport, others cannot as Gametest is failing to detect their message

Expected Result:
All players should be detected in the Gametest Framework's chat detection

Observed Result:
Chat detection is highly inconsistent and seemingly random as to which players it listens for.