So I did some extensive testing and discovered the issue lies with the Forward Shading method, SSR, and Stationary Skylights.
We made changes to the lightmap mixing for roughness as you know, but we also made minor changes to the Skylights intensity which now behaves differently. There was a small oversight in adding this change to our 4.14 release notes which has been addressed in internal comments for the bug below.
The effect you achieved in 4.13 was actually based on an incorrect skylight intensity and SSR working within its limitations with forward shading. You can tell because if you disable SSR on your material, the effect is gone. You have two options at this point. You can stay on 4.13 and work with the process you have established, or use 4.14 and work with the new behavior of Skylight intensity for environment reflections and the improved SurfaceForwardShading Lighting Mode.
Let me know if you have further questions.
Thank you,