After some additional research this issue seems to be the result of how lightmaps are baked on different threads. That means you will get a better result with a single mesh rather than many smaller modular pieces. Here is a work around that will allow you to use you modular floor pieces:
- Select all of your meshes that make up the floor.
- Open the “merge actors” tool (see screenshot for location)
- Make sure generate lightmap UVs is checked (should be on by default) and merge all of the selected actors
- Rebuild lighting
Note: Once this is done you may not be able to undo so please save a copy of your map before trying this.
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