Monitor KA272 bi with AMD Radeon RX 7800 XT, doesn't go any longer to 75 Hz. the screen goes black.

Hi,
I'm using the KA272 bi monitor since about 1 year with an AMD Radeon RX 7800 XT, directly connected with an HDMI cable (quite good quality).
Since some weeks I encountered an issue: the monitor doesn't go any longer to 75 Hz.
When I try to go to such framerate, the screen goes black…. (and after some seconds it comes back to 60 or 50 hz… depending on the starting value).
I tried to do the same operation both with pixel format RGB and YCbCr… and still nothing solves.

I've tried to reset the monitor (reading some hint in the forum), i reinstalled the drivers, both for the monitor and the Radeon…. but still i cannot switch into the 75 Hz.

How can I fix the issue?

[Edited the topic title to include the topic issue.]