How to reproduce:
1. Download and open attached project "TestScene"
2. Load Scene "test" found in Arts/Scenes/Scenes
3. Go to Lighting Window and click Generate Lighting

Expected result: Seams between planes are not visible when shadow is rendered
Actual result: Seams between planes are visible and shadow is distorted

Reproduced with: 2018.1.7f1, 2018.2.0b11, 2018.3.0a3
On version 2017.4.6f1 the issue is even more severe
Does not reproduce with older versions as 'Shadowmask' and 'Distance Shadowmask' modes are not supported in preview version of Progressive Lightmapper

Note: Seams are also visible using Auto Lightmapper Filtering setting

Update: The filtering setting cannot filter across objects.
Workaround to lessen the issue:

1) Author the floor as a single object in areas where there are shadows and other abrupt lighting changes.
Place the seams in places where it won't be seen (e.g. along dark areas in the texture).
This is typical for mobile.

2) Use real-time shadows in the near field - either `Indirect Only` or `Distance Shadowmask` modes.
This is typical for Consoles and PC.