The answer is KEYFRAMES! I had been rendering to uncompressed video and that was the problem. You can't store keyframe info in uncompressed video. I switched to Microsoft's codec and inserted keyframes every 10 frames (1/3 second) and now it works fine.

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System: Lenovo Legion Pro 7 16IRX9H Laptop | Windows 11 Professional | 32GB RAM |  14th Gen Intel® Core™ i9-14900HX | Nvidia RTX 4090 Laptop GPU 16GB 9728 CUDA Cores
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