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I was hoping that it was a bug, we never know with Jagex though. I just had a look through the patch notes and I didn't see where they had updated the chat. I literally just logged in and seen the chat and was very annoyed by it. I can see that it wasn't intentional, but needs to be fixed lol.
There was an engine update (hence why it was not in patch notes) to fix a bug with text colouring in certain use cases. This is a side effect of that change and something we're investigating, do not fear, the bug will disappear! No ETA.