Z fighting in built game - since upgrading to Unity 2018.3

I have a commercial project that previously was running on 5.6 and we recently upgraded to Unity 2018.2 and just very recently 2018.3.

Since upgrading to 2018.3 we have serious z fighting issues in scenes that on 5.6 had no issues at all. What's more the problem does not occur in the unity editor it only surfaces in a build.

I can't find any problem reports or posts about this, has anyone else experienced z fighting problems and can shed any light on what is going on? I've not managed to find any bug reports.
Of course we can try to separate materials more but as this was not an issue in the past I'm more inclined to believe this is a shader issue or something in the new builds.