Question....why would you drop a round in the chamber and then drop the slide on it. Most every gun I have messed with is not designed for that.
The extractor is not intended to be pushed over the lip of the round. This damages it.
Guns with bending extractors it's an absolute no-no because you have to bend the part to do it. guns with articulating (hinged, pivoting) extractors it's a no-no but not as bad. As you've seen it can be done, maybe even many times, but there are a lot of things that will tolerate abuse for a period before breaking.
If you look at the breech-face of a gun you can see whether it can be closed on a chambered round. If there is a raised surface all around the rim (preventing the round from sliding), it's OK. If there's a groove along the path of movement of a round, it's meant to slide up under the extractor, not for the extractor to snap over it.
A couple examples I have from the last time this came up: https://imgur.com/a/jPiXl