This has been happening frequently to me recently. I always have to restart the device to get back the ability to take screenshots via vol down + power. Pixel 8a, currently on build number 2025011500. If anyone has any further information on how to fix this without having to restarting the device, I would be glad to know.
The phone doesn't even have to be running a long time. It happened to me again on 10 hours uptime after and update today.