I have what I believe is part of the same issue, combining several tarplanar maps within a layer shader produces incorrect results.
I have done several tests and the issue can differ, and restarting everything has no effect. On diffuse, the result is completely black. Mixing a base color with one triplanar map works correctly, but adding and combining an additional triplanar map screws everything up. On the reflection glossiness, some details are rendered, but incorrectly, apparently some parts of the maps got inverted. Again, rendering with only one triplanar in the glossiness produces correct results
In either case, the render is not accurate, the material preview is not accurate (matches render), but the preview of each specific channel is correct and as it should render, which only enhances the mystery for me
Attachment 1 - Diffuse: Base color mixed with one triplanar map in a layer shader. Reflection: One triplanar map on each channel. Correct results
Attachment 2 - Diffuse: Added another triplanar to the layer shader. Reflection: Off. The preview in the diffuse is correct, the overall material preview and render are incorrect.
Attachment 3 - Diffuse: Same as attachment 2. Reflection: Added another triplanar map in the glossiness by mixing in a layer shader. Again, the preview in the reflection is correct, the overall material preview and render are incorrect.
As these things often happen in the middle of a project, feedback is highly appreciated