If you ever launched into a match and got stuck on the loading screen, with the game clearly running underneath but no way to see or play it, this patch is for you. I'm sorry. That is a rotten first impression, especially during the festival, and far too many of you sat through it.
I finally tracked it down. The freeze only happened on HDR monitors with Windows HDR turned on. At startup the game was switching its output into HDR mode, and on those displays the screen locked onto the loading image even though the match had already loaded behind it. I thought earlier updates had solved the loading screen, but there was a separate cause hitting anyone on an HDR display, and that is what this patch fixes. It never appeared on my own machine because I was not testing with HDR on, which is exactly why your reports were what cracked it.
The loading screen no longer freezes on HDR monitors. I turned off the game's HDR display output, which was the cause. The game looks the same as before; it simply no longer hands the screen to the HDR path that was hanging.
Still on an older build, or hit it before this update reaches you? Turning Windows HDR off (Settings, Display, HDR) clears it right away.
Thank you to everyone who sent reports, logs and screenshots, and especially to whoever spotted that Alt+Enter brought the screen back. That one detail pointed me straight at the display path and led me to the cause. I could not have found this without you.