Tried testing on other servers and singleplayer in 1.15.2. /tell tab autocomplete works immediately, but it doesn't on Cubecraft.This is not a bug! It's how the command system works after 1.13. Post 1.13 command works by guessing the commands. This happens mostly client side, some commands that exist on Cubecraft do not exist in Vanilla Minecraft thus do not work completely for every command. The /tell command included, also for /report. If this a problem just switch to 1.12.2 or earlier version of the game
Why'd they change that? Also, if the command is client sided, why would the client care that the command was changed server side? As far as the client would know, the /tell command is still the same, right?I know that the /report command doesn't auto complete because of this. The /tell command is part of Vanilla Minecraft, but Cubecraft has changed it so that when you use it displays a message "Use command like this, /msg" This means the command has been altered and Minecraft doesn't know how to auto complete. You can see that it just says [argument] and not [player] [message]. In the other server where you tried it out, they most likely didn't change how the command works.
Hopefully this answers your question better!
~Fesa