The resource pack attached to MC-212630 offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.

The bug

Melon and pumpkin stems appear considerably darker in-game than they should due to missing ["shade": false] parameters in their model files.

How to reproduce

1. Plant melon or pumpkin seeds
2. Have the seeds grow to their full size
3. Notice how the stem looks somewhat dark. If you look at the point where the two planes intersect, a clear boundary can be seen where the shading changes, which does not apply to other crosses. this can be seen to not be a texture issue by looking at both of the planes individually, which reveals they are lit differently.

Current appearence:

Expected appearance:

How to fix

The resource pack attached to MC-212630 fixes this issue completely, alongside fixing multiple other issues with the stem model.

Notes

While this was one of the cases listed in MC-68302, a 2014 ticket which was resolved as Works as Intended, I'm still reporting this independently of it for a multitude of reasons:

• This bug makes melon and pumpkin stems very inconsistent with all other crops, as well as all other cross-shaped plants, as absolutely none of these use shading.
• This behaviour is an unannounced change from 1.7 and prior versions, as these all had the stem render without this problematic shading.
• MC-68302 lumped several different model issues into one ticket, which is very confusing, and a discontinued practice which is generally not recommended much in more modern times. The individual issues in such tickets deserve a chance on their own as to allow for cleaner bug reports, fixes, communication and feedback, analyses, and so on.
• Two other issues split off from MC-68302, namely MC-129826 and MC-214662, are still considered valid issues.
• It just looks downright unsightly.