Entering commands at the end of a /execute command in spigot 1.13-pre7 does not tab complete, where it does in vanilla.

For example:

/execute run say hi

Typing the above command under spigot it will tab complete the command name and the first argument, but "say" and "hi" will show red and there will be no tab completions shown whilst typing.

In vanilla, tab completions will be shown for all 4 words with appropriate coloration.

The above simple command is just an example, I realise it is practically useless without the other options for /execute. It behaves the same in other examples:

/execute if entity @e[type=armor_stand] run say True!

will show tab completions all the way up until "say True!", which will be in red. Works as expected in vanilla.