Create a LocalScript with the following code, then run the game and see how the autocomplete works:
local TextChatService = game:GetService("TextChatService")
local TCC = Instance.new("TextChatCommand")
TCC.Name = "TestUpperCommand"
TCC.Enabled = true
TCC.PrimaryAlias = "/testUpper"
TCC.Triggered:Connect(function() end)
TCC.Parent = TextChatService
You will find that the command will always autocomplete up to the first capital letter in the alias, then once that letter has been typed (in either lowercase or uppercase), autocomplete will no longer work.