Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.19
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-02-12 16:20, hash: rB77d23b0bd76f
Short description of error
A mesh with the Weld Modifier gets incorrect vertex and UV values (in some situations) on vertices welded by the modifier. When the object has multiple vertex color and UV sets, the error manifests identically on all of them at the same time.
This is apparently a known issue, mentioned as a to-do here:
It may be additional information to note that UV channels values are also affected.
Exact steps for others to reproduce the error
Image 1 - a vertex color set with only a single orange color present:
Image 2 - vertex color after weld modifier, please note black vertex colors on merged faces:
Image 3 - UV set used to store per-vertex ambient occlusion:
Image 4 - UV set after weld, with the identical dark regions: