Hi all, thanks for the replies, unfortunately I can't update often as I only have access to the computer on show days, and have limited time to troubleshoot.
I did some more tests after updating drivers for the Declink to latest version, with no change on image quality on Isadora. I later checked on Blackmagic Media Express and quality was OK, as expected.
Source looks good on any software I use, problem only appears on Isadora while importing via Decklink, no matter the resolution, frame rate, interlaced or progressive scan, or device used as source. When capturing the same signal into Isadora via a HDMI to USB device (tested with an ATEM Mini Pro and a Roland V-02HD Mk2) image looks ok, unfortunately due to production requirements I'm forced to use the Decklink, so no luck there.
I'm at a loss here, only things I can think of is to update video drivers, which I will do next time I have access to the computer, and to enable/disable hardware acceleration; if that doesn't work I don't know what else to try.
Again, many thanks for the responses and suggestions.