No it doesn't. If the content of a url changes then the only way to have reproducibility is caching. You tell nix the content hash is some value and it looks up the value in the nix store. Note, it will match anything with that content hash so it is absolutely possible to tell it the wrong hash.

Apparently I made note of this in my laptop setup script (but not when this happened so I don't know how long ago this was) so in case anyone was curious, the jar file was compiled with java 16, but the nix config was running it with java 8. I assume they were both java 8 when it was set up and the jar file upgraded but don't really know what happened.