Experimental starts games in a frozen state

The latest Experimental has the following effect on three games that I've tested, maybe all my collection. Proton 8.0-3 is fine.

The game seems to initialise OK when I press Run. However, the very first thing that the game shows will be frozen in time, sometimes it will be frozen in a faded state. For example, if the game normally fades in a logo, the first thing I will see is that logo, frozen, at some stage during fade-in.

For example, Ori and the Blind Forest: Definitive Edition usually first shows the Ori character running on the spot as the game loads. With Experimental, I get this static (only darker than shown here, because I could only get this screenshot by using Proton 8.0-3):

The same goes for INSIDE (darker again, this screenshot was taken using Proton 8.0-3 when the game works fine):

And with Euro Truck Simulator 2, which apparently doesn't fade in, but rather immediately shows a background image before loading the main menu. This time the background image appears, frozen. The mouse cursor is unresponsive:

In all cases I have waited a few minutes. Sometimes I'll catch the screen moving a bit, as if showing the next frame, but again, frozen. Shift-tab and F12 (for a screenshot) are unresponsive. I am able to alt-tab to other applications, and I can kill the game with alt-F4. I'm using Openbox in XFCE on Void Linux. I think this started with the Experimental version before the 10th August update.

I attach PROTON_LOG files for both working (8.0-3) and Experimental states.

steam-227300-8.0-3.log
steam-227300-experimental.log
steam-304430-8.0-3.log
steam-304430-experimental.log
steam-387290-8.0-3.log
steam-387290-experimental.log

Start by comparing the attached steam-227300, steam-304430, and steam-387290 PROTON_LOG files for Proton 8.0-3 and Experimental, then reproduce the freeze in the three named games. Done means the initial game screens display normally, continue moving, and accept input under Experimental.