Hi Chrisio,
Interesting, exactly the same thing here when we had the machine brand new 3 years ago, not a single problem until recently it was only doing it on blue bottom sheet first for me. I did have a few engineers out and they said it was the baffles causing it, so I had them replaced and it did disappear, however it still does the odd occasional one now. The other sheets I just get a really tiny bit now on my NCR. I don’t have a stacker but I run from trays 6 and 7 if I’m doing duplicate and then tray 5 if I do triplicate.
The only other material I get dog earring really bad on now is my DigiTuff waterproof plastic on 195micron. So I’m having to use the 295micron version. Again I never use to have this issue before.
Apart from that uncoated and coated stock is not a problem so far.