In case it helps anyone else, I had a similar problem with a different HDMI display, a 5" with 800x480 resolution. For my application, I was using DRM rather than Xorg/Wayland for display rendering, so I didn’t have the option of using a custom modeline. I ended up addressing the problem by specifying a custom EDID with a kernel boot parameter, overriding the display’s reported EDID. More on that here.