I installed spark on my server and typed /tps, the tps message it was supposed to show did not show up. When I disabled "handle system messages" and tried again, it did.
I think the reason why this happend is that these messages starts with a symbol wrapped in bracket. Somehow ChatHead recognizes the symbol in the bracket as a username which is not. I think it would be more reasonable for the message to show without a head icon instead of not showing up at all. It should also be possible to disable head icons for illegal usernames with regex.

Is there any error in the logs?
This seems like a very similar issue to #90 of the new heuristic crashing the message handler, preventing the message from being printed to the screen.
#90 was fixed in 0.12.1, so make sure you're using the latest version.

Gonna try installing spark and see if I can invoke the issue but I'm afraid I need more context to be able find it.

Hm... no luck on replicating the issue so far.
Used No Chat Reports to ensure all messages are server messages and the heuristic is always being used.

Either it's the Chat Heads version you are using or there's still some unknown issue with the heuristic and certain names.

edit: I've been fuzzing the heuristic for an hour now, no errors so far. Maybe it's something outside of it, but the Chat Heads version being out of date feel like a pretty safe bet.

Is there any error in the logs? This seems like a very similar issue to #90 of the new heuristic crashing the message handler, preventing the message from being printed to the screen. #90 was fixed in 0.12.1, so make sure you're using the latest version.

Gonna try installing spark and see if I can invoke the issue but I'm afraid I need more context to be able find it.

I got this problem in 0.12.0. Updating the mod solved the problem, thank you.