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
Related issues
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Apologies, I accidentally deleted your picture. I uploaded one of my own.
Can confirm, definitely a difference between 1.15.2 and 20w21a.
Confirmed/Reproduced in 20w28a.
Can confirm for 20w46a.
Can confirm in 21w06a.
Can confirm in 21w13a.
Can confirm in 21w16a.
Can confirm in 1.17.1.
Can confirm in 1.18.2 and 22w15a.
[media]Can confirm in 1.19.
can confirm in 1.19
Forward-resolving as a duplicate of MC-262644 for the description and attached pack.