Created attachment 105017 [details] example of pen pressure before and after alt-tabbing When switching out of a window or losing focus of Krita, the pen pressure disappears entirely. Turning off then back on pen pressure does not fix it either. Pressure does not return until a program restart. On Huion H690 graphics tablet, Windows 10.
Sorry, but that's a bug in the driver, not in Krita. We've had similar reports for Wacom and Genius -- and we've seen reports like this for other applications, like photoshop, as well.