Hey rehkram, I've just tried a clean install of Sonoma, checked the monitor res, also tried to switch to 75 Hz, settings seem ok. Unfortunately the graphic glitch involving Notes happened again, this time also on the built-in display. I will wait for further versions of OCLP, that hopefully could solve the issue, meanwhile I will run Ventura as main OS, also because feels like it runs better on my Mac.Very strange. I guess you've checked your display resolutions and refresh rate are correct for the monitor? Many times after an update I have to mess with those settings. I run my Dell DisplayPort link at max res 2560 x 1600, 60 Hz, and the internal display Apple default res.
I don't know whether it's the case, but in the past I experienced a different graphic glitch that randomly showed green squares on images (on Safari and Photos). Can't remember exactly when, but I think I had Big Sur running at that time. Some said it would possibly be a hardware failure of the GPU (don't know which one). I was thinking about this, wondering if in some ways could be true and involved in triggering the latter graphic glitch. Have to say that the green squares never happened again though. Anyway, thanks for your suggestions!
EDIT: right after posting my reply, I discovered that is not Notes in general that trigger the issue. If I'm browsing "regular" notes nothing happens, but if I open a list-like note, the glitch happens. Unfortunately I have some of those.