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Is this background terrain from 3dsMax animated and saved in files per-frame?
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Hey, Ambrozie,
I take it you're in a hurry, and you probably don't feel like uploading files yet again so here's what you can do in case you'd rather not send the hip file:
0. Copy your working scene and rename the new one to debug
1. Go to one of those frames where the geometry is missing and run a single frame render to verify it doesn't only happen when rendering a sequence (that's very unlikely)
2. Hide all objects except the one that disappears
3. Render again to verify the problem is still present
-> now that everything else is hidden, can you please check the alpha just in case ?
4. Copy the V-Ray Renderer ROP and apply some very low-quality render settings - GI disabled, bucket sampler subdivs set to min/max 1/2, camera resolution override to 640/360, etc. Do not change any motion blur settings.
5. Give it another render - the problem should be still there
6. Disable motion blur on the camera - the problem should disappear, judging by the video above
7. Enable it again and try setting the "Num.Samples" option in the Object Properties of the Geometry node holding the disappearing geo to 2 - default is 0 which means "Use the ROP's Geometry Samples" - IIRC those were set to 6 in your scene.
8. If that doesn't help, try increasing the Shutter Speed on the camera to 100 or so - the scene will go dark but if the geometry shows up in the alpha, it means V-Ray is not finding the cache files to sample for motion blur
9. If you're using alembics, try switching the Alembic SOPs Load method between "Delayed Load" and "Unpacked" - if that changes something, it's probably a bug we need to fix
10. Lastly, the Geometry node's Sampling -> Geometry Velocity Blur method - you could try switching it back to "No velocity blur" to see if that resolves the problem
V-Ray is not doing anything fancy when generating motion blur - it just needs start and end geometry in the case when not using Velocity Blur, or an attribute when it's being used. The parameters mentioned in points 7-10 are what affects how this happens.
I tried reproducing this here with the files you had given us a while back but no luck
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