When I run some simple proton-j code to open a connection while my machine has no internet access, proton-j appears to leak file descriptors. I have some code attached that you can use to repro this issue. You simply need to run the sample while your internet is disconnected.
From what I can tell, proton-j does not leak any file descriptors for successful connections.
I've been using Window's to process explorer view my machine's file descriptors over time. This same issue is present in Linux and MacOS as well.