> you might not be aware of when a mutation occurred

Yeah, this is a risk with any language that mixes some functional features/style in with non-functional features. Rust does a pretty good job of letting you mostly delineate the two, though there are still escape-hatches

I've actually been toying with a language design that has both, but draws a hard distinction between them. We'll see how it works out :)

But most of the time, if you're not using a totally pure functional language, functional programming is more a practice/style/ethos/feature-set than a hard constraint