Basically when doing :w does neovim behave differently then vim?
i have this strange problem with trunk (a rust webassembly file watcher and page server etc)
that when i save changes with neovim trunk does not detect them (with --clean, and on stable and nightly)
on the other hand if i save them with vim it does detect changes.
related: Trunk does not detect changes if files saved with neovim · Issue #331 · thedodd/trunk · GitHub
It could be related to the backupcopy, does the behavior changes if you switch the option?
I was having the same issue with kaocha (a clojure test runner): Watch mode not working with neovim · Issue #277 · lambdaisland/kaocha · GitHub
I found the vim docs about backupcopy a bit unclear, I liked this explanation more: Vim #105: backupcopy - YouTube
setting backupcopy to yes solves the issue for me thanks you’re a live saver 
pretty sure there is a bug in the core
when neovim is started with the default settings auto it will not work
if you set it in the options to yes it will work but if you then set it manually to auto it still works
it just does not work upon booting up neovim with the default setting set
edit: not sure anymore if it’s true now it works just as expected even without setting backupcopy
really strange