But it IS rendering forward only shaders even if camera is in deferred mode - and it should - because all the semitransparent objects (and all other special cases with translucency etc) can only be rendered in forward only mode and it works fine. Except when using the View Dir (tangent space) or World to Tangent matrix nodes AND turn on tesselation - then it still renders the shader, but the values from these nodes are not valid. The rest of the shader is fine - but as soon as you plug in these nodes, you can see the visual errors from the bad values. I suspect the bug is in the vertex shader where some values are not interpolated correctly when tesselation creates new vertices (This is clearly visible on the geo - as some of the polygons have valid values (original vertices I guess), but the rest has invalid values (new vertices from tesselation)).
Yes no doubt something is not being correctly calculated/valid.
I'll dig a bit deeper and do some tests to compare both compiled versions in forward and deferred of a shader using those instructions and see what is the issue internally and hopefully we can prevent this from happening on our end.