Hey, wasn't sure if you were aware (checked the github issues but I didn't see any mention of this) Both DirectX and OpenGL style normalmaps generated in this are wrong. Not sure about the packed normal maps (2 channel normal map variant labeled as "default" in this program) are correct. I imagine they are wrong, but not entirely sure. Anyway, both DirectX and OpenGL style normal maps have the red channel represent the right side of the normals, meaning if you have something that protrudes outwards like bricks the red channel for each brick should be on the right, that in not the case in this program. They are on the left side. Super easy fix for people making textures in this: Use the decompose node and invert the red channel (top) and then recombine using the combine node.
That format parameter in the Normal node should not be used anymore. Just let it generate the "default" normals, which should render correctly in the preview. Correct normals are then generated when exporting for each engine.
And yes, the tangents on the preview are wrong (was an issue in Godot's default meshes when I started Material Maker, and fixed since then).
You can indeed trivially modify the normals format as needed using Decompose and Combine, but I'd recommend SwapChannels that is easier. And if you need new export options, please ask. :)
Is the issue with broken tangents on Godot's primitive meshes reported on GitHub? This was supposedly fixed in Godot 3.1: https://github.com/godotengine/godot/issues/16009