The bug

With the experimental deferred rendering, the X- and Y- axes in normal maps are expected to be lit from a different side. The result is that the game applies the shading on the opposite side of the texture. Flipping the red and green axes remediates the issue.

Steps to reproduce:

• Download and install the attached resource pack.
• Create a new world with the Render Dragon Features for Creators experimental toggle enabled. Ensure that the Graphics Mode (in Settings → Video) is set to "Deferred Technical Preview".
• Place down some cobblestone and end stone within a source of point light or directional lighting. These two blocks have the same normal mappings assigned to them, with end stone's being rotated 180°.

→ Cobblestone appears shaded from the reverse side, using a mapping of +X, +Y, and +Z.
→ End stone appears shaded correctly, using a mapping of -X, -Y, and +Z.