Probably related to MC-51447, but it's marked as fixed. This issue can most commonly be seen with sloped/slanted/tilted rails.
Position of block models affect it's lighting what creates rough color transitions
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Forward-resolving as a duplicate of MC-262644 for the description and attached pack.
Apologies, I accidentally deleted your picture. I uploaded one of my own.
Can confirm, definitely a difference between 1.15.2 and 20w21a.