Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce RTX 2070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.64
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-07-18 10:55, hash: rB687f1df5dbe2
Short description of error
With Cycles Motion Blur enabled, intermittent frames on a rigged mesh are rendered with stretched/ripped triangles.
This is probably related to other Cycles Motion Blur reports, but I've spent about 6 hours troubleshooting this and am hoping that there is some information here that can help.
I would give a list of "bad frames" but I've found that sometimes the same frames will render OK, and
other times, they are broken. Additionally, a frame may render with artifacts - I could advance the timeline render another frame, then go back to the "bad" frame without making any changes, and the frame that was previously "bad" would render correctly - i.e. a frame that renders with artifacts may NOT always render with artifacts - something else is in play here.
Some things that have helped "fix" frames:
Turning off motion blur fixed every frame I checked
Turning off the Multi Modifier Armature "Preserve Volume" - this does not always work
Triangulating the entire mesh - this helped a lot of frames, but didn't always work
Adding or deleting geometry may fix a particular frame, bud didn't always work
Changing the motion blur duration may fix a particular frame, but didn't always work'
Changing the motion blur position (start, center, end) didn't always work
Additionally, triangulating the mesh improved shading on quite a few frames.
Some things tried that had no effect:
Changing materials didn't fix any frames I checked.
Changing lighting or HDR environment didn't fix any frames I checked
Rendering from a different camera on a frame with anomalies, the image still had anomalies
Moving the camera had no effect.
Exact steps for others to reproduce the error
Open the attached project (sorry for the size). Render out maybe 20-50 frames - you will get at least one frame with issues.