Hi I have a kind of same problem. A monitor able of displaying at 1920x1080. Intel HD Graphics 620 with driver i915/modesetting.
With old Stable (kernel 4.9.0-9) it was working fine. With new Stable (kernel 4.19.0-6) it set max display 1024x768. See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=892939 White waiting for a solution you can boot new kernel with "nomodeset" kernel parameter, then X will use vesa driver, and it is better than nothing, but not good for heavy graphics. Alternative keep kernel 4.9.0-9. Brgds Torben