What about being treated as code when it's supposed to be?

(What is the difference between code execution vulnerability and a REPL? It's who is using it.)

Whatever you call program vs. its data, the program can always be viewed as an interpreter for a language, and your input as code in that language.

See also the subfield of "langsec", which is based on this premise, as well as the fact that you probably didn't think of that and thus your interpreter/parser is implicitly spread across half your program (they call it "shotgun parser"), and your "data" could easily be unintentionally Turing-complete without you knowing :).

EDIT:

I swear "security" is becoming a cult in our industry. Whether or not you call something "security vulnerability" and therefore "a problem", doesn't change the fundamental nature of this thing. And the fundamental nature of information is, there exist no objective, natural distinction between code and data. It can be drawn arbitrarily, and systems can be structured to emulate it - but that still just means it's a matter of opinion.

EDIT2: Not to mention, security itself is not objective. There is always the underlying assumption - the answer to a question, who are you protecting the system from, and for who are you doing it?. You don't need to look far to find systems where users are seen in part as threat actors, and thus get disempowered in the name of protecting the interests of vendor and some third parties (e.g. advertisers).