Hi
This is probably caused by denoise (profiled). It is very hard to make a perfectly accurate preview in the case of denoising, so it is better to look at the image at 100% when denoise profiled is activated.
OK. I did a few more test. Apparently, this “bug” isn’t there without OpenCL. But with ocl it is even more apparent when I go up to 500% with detail in local contrast.
I guess, darktable folks are busy with getting the release ready, judging based on the activity on github. Maybe you already did, but if not, and you still consider your observation a serious bug (after considering the input you got in this thread), think about filling a bug report.
I did a few days ago. One user could reproduce. I guess the dev of this module is occupied with other things… I hope he will fix this eventually, not expecting an instant fix