Clarifying wave-particle duality in the double-slit experiment

My understanding of the double slit experiment is that the electrons (or photons) act as waves unless you observe which slit they pass through, in which case they act like particles. If you don't observe which slit they passed through then you get an interference pattern on the back screen, which shows that they're acting like waves. But if you do observe which slit they pass through then you don't get an interference pattern on the back screen, which shows that they're acting like particles.

I'm getting this info from none less than Richard Feynman himself:

https://www.feynmanlectures.caltech.edu/III_01.html#Ch1-S1-p3

Incidentally this is what I find on Wikipedia (emphasis added):

Interference patters are created by waves, aren't they? Doesn't the presence of an interference pattern demonstrate that the electrons are behaving in a wave-like way, while the absence of such patterns demonstrates that the electrons are behaving in a particle-like way? That's what I always thought.

Yet I just had a very confusing conversation (see [here](https://old.reddit.com/r/explainlikeimfive/comments/1ohop4w/eli5\_if\_everything\_is\_fundamentally\_waves\_at\_the/nm2phtb/)) with someone who insists that electrons do not change "states" at all, and they never switch from "wave mode" to "particle mode" or vice versa, because they're always in their original state. And I've found comments on this sub agreeing with this.

But if that's the case, how do we explain the double-slit experiment?

I feel like I'm being told that electrons definitely *don't* change their behavior and also they definitely *do* change their behavior. On the one hand, they definitely don't switch states and they are always wavefunctions and they never become "more particle-like" or "more wave-like", and other other hand they definitely *do* produce interference patterns (in a wave-like manner) or not (in a particle-like manner) depending on whether or not they're observed.

What on earth is going on here?