This bug  in the Valve github tracker seems to be caused by the hid-steam driver, written by me, introduced in 4.18.
With this driver there is a new input device, which is disabled when the Steam client is running. That is usually harmless, but some games go nuts with that extra device.
I have written a patch, that instead of disabling the device, removes it when Steam is running, and recreates it when Steam is closed.
I have submitted it to LKML a few days ago , but it looks like it went through the cracks... It would be great is someone could check it.