Just a guess. I've seen this with other devices. As this likely happens regardless which HDMI input is used, I would look at the output which is only one. While hot plugging HDMI, one of the TMDS transmitters could be partially damaged (blown one out of two in a differential pair). In such case I measure resistance from each of the TMDS outputs to GND at high resistance range range set on the multimeter, like 40 megaohms. If one of them have abnormally low resistance compared to the others, means that transmitter is blown. At low resolution this still may work. The other possibility is something damaged related to SPD lines, which causes impossibility to read EDID. In such case case it also may still work at some low resolution, depending on the hardware used. Of course one should check if there is no damage in HDMI connector and try changing the cable first.