Is there a name for this general class of problems? Where you have an accidental parsing/scoping error because the encoding format is ambiguous, or doesn't take into account a specific configuration of the message contents?
I would classify this as using an in-band signal rather than an out of band signal. Twitter used to have this same problems ages ago. I don’t remember the details but it was something like DMs being standard twitter messages with “/dm” at the beginning.