Indeed very weird.. I've never had that happen before on any monitor over 20+ years either. I'd be interested to see what the cause/solution is. Possibly the cable? I know I've had bad cables that worked with one graphics card and not another cause weird ghosting -- but it looked wayyyy less than that. That's almost like smearing.

When I usually had issues like these, it was mostly caused by a bad cable. Replacing it with a better, high quality cable usually cures the issue. Sometimes, a bad LCD panel could also give that smudged effect, I witnessed that on an old AOC 14" flat panel of a customer: no matter the cable I put, the picture always looked horizontally smudged. No difference between switching from the onboard graphics chip to the external graphics card, he always had the same issue so he just scrapped the monitor and got a new one.

Try to replace the cable, see if the picture improves.

Yup. Apparently it was a bad signal cable after all. I was sure I used different cable with 17" BenQ on a meetup where I got the card, but I'm not so sure right now... I don't remember either because... you know... beer.