Steps to repro:
1. Open the attached project and 'Spotlight_Tunnel_PLM' scene;
2. Clean baked data and GI Cache;
3. In the Lighting window press 'Generate Lighting' button;
4. After the GI is baked in the Scene view expand the Editor visualizations dropdown and select the 'Baked Lightmap' view.

Actual result:
There are seam artifacts on the mesh lightmaps in the areas where the meshes connect. See attached screenshot.

Expected result:
There are no seam artifacts on the generated mesh lightmap.

Notes:
- Reproducible in 2019.3.0a1, 2019.2.0a14, 2019.1.1f1;
- The seam artifacts are mostly visible when baking with Progressive lightmappers (CPU and GPU), almost not visible with Enlighten;
- The issue reproduces on both modular mesh and combined mesh (however, the combined one has different artifacts, see the screenshots, please);
- Increasing samples count doesn't fully solve the seam problem;
- Adding filtering removes part of the seam artifacts, but doesn't remove it fully.