Hello Steffen,
My apologies for the delay in getting back to you on this. I see your support message as well—thank you for the information there. You have an interesting project here, and I hope we can improve the artifact problem you are seeing.
One thing I noticed about your data is that it is very large—some number of times bigger than world scale. This user recently had a problem that seems somewhat similar, and they were able to work around it by changing the scale of their model. (Although, in their case, the scale had to be increased instead of decreased.)
Which option did you select for What kind of data is this? when uploading? I would recommend trying 3D Capture, if previously you selected a different option.
You could also try disabling Draco compression and/or reducing the maximumScreenSpaceError of the tileset (which, while probably not a real solution due to the performance hit it would cause, could help us diagnose the problem).
I am testing these options out on my end and will let you know if any of them is successful in improving the visual quality of the asset. Please let us know if you try any of them and have any findings to share.
Thank you,
Matt