Artefact when exporting JPG photo when B&W enabled

Hello,

I am running C11.3.1.6 (but it also happens with other older versions of C11)
I am having the following artefacts at export issue on one specific photo where export with long edge scaling and B&W enabled don't got well together.
My current DB contains other B&W photos that have been exported with the same process without any issues.

That's my current variant:
https://image.noelshack.com/fichiers/2019/44/3/1572452910-screen-shot-2019-10-30-at-17-27-53.png

If I disable B&W:
https://image.noelshack.com/fichiers/2019/44/3/1572452910-screen-shot-2019-10-30-at-17-28-19.png

If I reset B&W:
https://image.noelshack.com/fichiers/2019/44/3/1572452910-screen-shot-2019-10-30-at-17-28-08.png

My process recipe has scaling to long edge (LE) @ 2500 px
As I reduce the scaling, it gets better. JPG output with LE @ 2600 is almost clean (one white tick). @ 2800 ticks are multiple but lighter (thinner).
The display in C11 is clean, even if I regenerate the preview. The quick proof is therefore clean even with LE.

OpenCL Auto/Never doesn't make any difference.

Any ideas?

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