Any entity selected by the /execute command does not execute the command it is supposed to execute. Always responds with: "Failed to execute 'some command' as someEntity", ie. "Failed to execute 'tell ceagle Hello World' as ceagle".

This applies to all selectors, all types of entities and - as far as I can tell so far - all commands that are being used in combination with /execute.

Using the exact same commands via normal console, commandblock or as an ingame command does work without any issues.

Tested with latest Vanilla 1.12.2: Rcon /execute works properly
Tested with latest Spigot 1.12.2, no plugins: Rcon /execute not working
Tested with latest PaperSpigot 1.12.2, no plugins: Rcon /execute not working (just for the sake of testing)

No Java exceptions at all, no additional information logged. But since it's 100% reproducible at all times - if you need any further information, I'll be glad to provide that.