When I try to use any plugin command after the run argument of the execute command the server says that the command is incorrect, as you can see.

It's like the execute command doesn't know about plugin commands, in fact they never come up in the execute tab completition (after the run argument).

To reproduce the bug simply put any plugin with at least one command into the plugins folder (restart) and try to use

/execute as @a run <a command of that plugin>

I also found this bug on spigot 1.13.x and 1.14.x