1. Create a simple level sequence with a moving ball
2. Add it to Movie Render Queue and then add the Anti-Aliasing setting to the configuration.
3. Set the temporal sub-sample count to 8.
4. Render the Level Sequence. The resulting images should have a nice blur between them.
5. Under the "30fps dropdown" in Sequencer, enable "Lock to Display Rate".
6. Render again. Inspect the resulting images, they don't have a smooth motion blur and instead look like stamped images
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-139967 in the post.