Missing Logs: Inconsistent and Unexpected I/O flush behavior

This is a Proton Wine counterpart of an upstream Wine issue I just filed (summary below) describing how, under some conditions, I observed application prints missing from a Linux terminal and/or I/O redirect. This made my recent debugging of https://github.com/ValveSoftware/Proton/issues/7529 much more challenging because I was working with limited information.

I found the behavior in Proton Experimental (9.0) to be substantially different (worse) from Wine 9.0 and 9.4 so I thought this should merit its own bug report.

In summary:

1. I put together a simple app that prints to stderr and is configurable to cover multiple corner cases.
2. In all cases, I found that there were some number of prints missing when running the app through Proton Experimental.

Full details here. I'm unable to make the drive fully public per my organization's policies so please request access.

Start with the linked upstream Wine issue and the provided reproduction details, then compare stderr output and I/O redirection under Proton Experimental 9.0 and Wine 9.0/9.4. Done means the missing-print behavior is reproducible and its handling is corrected or clearly documented with verification across the reported cases.