What I think of is to use some transparent panel over the display, which shifts between transparent state and black state, it must be fast, it must be durable, it must be capable of many cycles, it must be very transparent, and it must be possible to time it strictly.
The most obvious would be...
Could it be a bug/regression in drivers, or perhaps a Windows 10 bug that nobody noticed? Maybe some other bug was fixed and accidentally broke 1440p@60hz. You could try older drivers and older versions of Windows/Linux to see if that works. Just some random speculation after not finding an...
https://tftcentral.co.uk/articles/when-hdmi-2-1-isnt-hdmi-2-1
This was kicked off by a Chinese monitor marketing their monitor as having two HDMI 2.1 ports. It was 1080p, 240hz, and used TMDS.
here's a quote from the article with imo the most important information.
I understand that this is...