Proton 4.11-8 gets disconnected on VAC protected servers

This is probably not affecting just one Source mod, and it might not even be related to Proton directly. This is to let Valve know about the issue, in case this is related to their back-end servers somehow.

Since the most recent Steam / Proton update, after a few minutes of playing, Linux users get disconnected from VAC protected servers when using Proton in the Half Life 2 multiplayer mod Neotokyo, as mentioned here.

The error message on the server side is "No Steam logon". On the client side:

Two other Linux users (one using Arch Linux like myself) are having the same problem, with 4.11, 4.2 and 3.16.

Here is a useless log.
Here is another log with +winsock enabled: long gist

System specs: gist

Reproduction:

• Install a Source mod (ie. Neotokyo)
• connect to a VAC protected server
• wait a minute or two, you'll get disconnected.

Native CSGO, Dota2 Linux clients are not getting disconnected by VAC.