This is pretty common. They usually work best with CRT displays.

Yes. Pretty common. And I see it on VLB cards as well. I have only had three ISA VGA cards, that did not produce these image issues. One being a cheap low grade Cirrus Logic CL-5420. The other two, were an ET-3000 with VGA and EGA output and then my Orchid ET-4000.

I second that. I believe it's no so much the fault of the graphics chips (it is digital internally), though,
but rather a bandwith limit and/or related to the used RAMDACs and the other parts installed on the cards..

Another reason that I *believe* that could be the cause are the stricter RFI/TVI regulations of the 1980s/early 1990s.
Back in these days, telecommuications authorities were much more conscientious than they are today.

In these days, all commercial computer/telephone equippment had to be properly shielded.
Violations had ben pursued and sanctioned.

By comparison, the mid-90s, when PCI cards showed up, the situation was incredible relaxed (for the better or worse).

Nowadays, even professionals do install unshielded hobbyist grade hardware..
Most devices don't even contain until-then elementary parts like removable fuses or an el-cheapo shield made of cardboard/aluminium foil. 😢