The bug
It's hard to know the observer block's direction from the side because the observer side isn't mirrored.
MC-108915
Wrong direction of the observer block side
Resolved
Invalid
5
(Unassigned)
Confirmed
Textures and models
Minecraft 16w41a, Minecraft 16w42a, Minecraft 16w43a, Minecraft 16w44a, Minecraft 1.11 Pre-Release 1, ..., Minecraft 1.11.2, Minecraft 1.12 Pre-Release 5, Minecraft 1.12.1, Minecraft 1.13.1, 1.15.1
Retrieved
Confirmed for 1.13.1.