A couple of weeks ago hidden prompt injections were discovered and we covered it at the time.
This video explains it in more detail, and also highlights implications beyond hiding instructions, including what I call ASCII Smuggling. This is the usage of Unicode Tags Block characters to both craft and deciper hidden messages in plain sight.
Using Unicode encoding to bypass security features or execute code (XSS, SSRF,..) has been in use for a while, however this new TTP enables more sophisticated attack scenarios.