"send client a chat line including §3 §6 §3 §6 §3 §6 §e or §3 §6 §3 §6 §3 §6 §d to disable radar and cave mapping respectively"
I suspect you may be able to have a command block push those out as "raw .json" to *test*. Be sure to send a simple color message *first* to make sure the command block is behaving as you expect "just to send a color message using raw .json." Once you've verified you have that right, try the codes separately. If the one works, the other still does not, it sounds like a bug to be reported. But you can definitively say, "Command block with XXXX" fails, while command block with YYYY works as expected." That sucks out any issues with any plugins at all, and it makes it very easy for anyone to test and verify you are correct even if they are using the Vanilla server.
EDIT: Or, come to think of it, verify it's a problem on Spigot and NOT Vanilla, maybe. A command block removes any issues with plugins, and it's a great place to start.