I found that sometimes I couldn't see the side texture of the obsidian, so I enabled a texture pack to check the problem. To my surprise, the original textures of all the blocks were darker in the X-axis direction than the others.
I found a similar problem in the Y+axis direction. When we look at the leaves from the bottom side, we can see that the leaves on the Y axis are too bright, and the texture on the X axis is darker than that on the Z axis.

I reported the bug about Y+axis issue in MCPE-170856 Top inside texture of Cherry Blossom leaves is all bright - Jira (mojang.com)

However, the texture problem is much more serious than I've found before, and all blocks have the same problem.