Wheat in corners grows slower than other crops

I placed 7 wheat in a U shape inside a square building.
What I expected to happen:wheat would eventualy all grow, mostly at the same rate
What happened: all the wheat except the two corners grew to full even after I played in the world for awhile after they grew.

To reproduce:
1. Place wheat in a U shape W=wheat E=flowing water L=ladder
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2. Build 5x5 building with door space around it.
3. Wait for wheat to grow.