1. Here's why

If you add a Motion Blur Effect to a Movie or a Clip, the motion blur on animated objects is visible on Photos and in Movies at all resolutions, apart from Print and Poster resolution.

This is a technical constraint.

The constraint is due to the fact that Print and Poster resolution Photos are rendered in a different way than Movies and Clips, i.e. using a tile-based approach.

In Lumion 8.0 and newer, the highest resolution that supports the Motion Blur Effect is Ultra HD (3840x2160) which is available on the Image Sequence tab in Movie Mode. This is the same resolution as Print resolution in Photo Mode. Please note that the Ultra HD (3840x2160) output resolution requires a graphics card with at least 6GB memory.

2. Workaround

2.1: To work around the Print and Poster resolution constraint, simply render a single Ultra HD (3840x2160) image via the Image Sequence tab in Movie Mode:

2.2: If you need a Poster resolution image, then rescale the Ultra HD image by following these instructions:

Knowledge Base: How do you render images at a higher resolution than Poster renderings?