Sometimes this happens when lightmaps are too large, or the lightmap override is set too high.
In your Unreal, go to Window > Statistics and sort the list by “Texture LM” so that the largest lightmaps are shown first, clicking them from the Statistics window will select them in the World Outliner and then down in the Details panel, scroll or search for Lighting and either lower the “Overridden Light Map Res” number or use this to override the size of the lightmaps on that particular model.
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