This ticket concerns an unannounced model change from 1.8; as said change is unannounced I've reported it as it may not be intended. I'm a tad more neutral on this case and whether it should be considered intended or should be fixed, however such a decision is ultimately up to Mojang. More information below.

The bug

In 14w25a, many changes were made to how custom models rendered which resulted in certain blocks and model elements gaining directional shading that should not have. Among those affected by this change was cocoa, in which the previously bright plane or "root" connecting the pod to the supporting log gained this darker shading.

While this issue is nowhere near as severe as other lingering 14w25a issues such as MC-129826, MC-214662 and MC-236474, I am reporting this anyway as this change was ultimately not formally announced, and many others like it have since been fixed. It also weakly constitutes a parity issue due to this not being the case on Bedrock Edition.

I ultimately find the current behaviour to be much more preferable, as it matches other similar blocks in the game, unlike the three issues mentioned in the above paragraph which result in major visual clashes. Ultimately whether this is intended is Mojang's decision - I'm just being the messenger. This is admittedly much less visually unpleasant than MC-249035, so I'd be less angry if this was fixed and not that.

How to reproduce

1. Place cocoa
2. Pay close attention to the thin connecting plane, and take a very close note of its appearance and brightness (screenshots for later comparison recommended)
3. Repeat this test in 14w21b or earlier (14w10a-14w21b recommended, as it'll appear much brighter than the main fruiting body)
4. Note how it appears much brighter in these versions

Expected results

The fully lit stem would remain so in current versions.

Actual results

It appears darker in modern versions.

How to fix

If this is determined to be a bug, it can be fixed by simply adding a "shade: false" line to the stem element in the cocoa models. A resource pack is attached to the issue which does this, with a comparison as follows:

Current vanilla models	With shading removed