If you program a register at a moment and in a way that causes two signals to "collide", the result effectively depends on transistor behavior. That in turn can be temperature dependent.
That was a fun rabbit hole. My first computer (8-bit, 4k RAM, 0.8 Mhz) had a video chip related to that 6845, the 6847, which was (sadly) a somewhat lower cost, less capable, less interesting chip.
For an example on the PC see https://int10h.org/blog/2023/03/cga-6845-crtc-phantom-vsync-...