Thank you for your replies.
Your suggestions pointed me to the more in-depth explanation of the problem here
https://forums.unrealengine.com/t/artifacts-found-when-tiling-uvs-with-clamp-sample-frac/227450
By disabling mips I was able to make this problem tolerable enough and mask out the rest of the artefact with a separate texture. It’s not the most elegant solution but at this point I’ll take it.
Again thank you for your support.