I've encountered an issue on macOS with libloading (which I suspect is actually a problem with dlopen).
To quickly summarize it: dropping and then reloading a dylib does not seem to update it if the dylib uses any of Rust's standard library.
You can see the problem by running, in this repo:
$ cargo run -- working # This reloads correctly
$ cargo run -- broken # This fails to reload
The only difference between the two variants is that the broken version has a call to print! and the working version doesn't. I've also reproduced this with other standard library methods, as well as once by creating an empty String.
I'm not able to test this on other platforms currently, so it may be limited to macOS, but it may not be.