GeForceLegend

These two do not relate to a problem. MC-204939 is caused by display value in item model file, but this one is caused by player animation.

This is caused by ambient occlusion of emitting opaque blocks are achieved by modifing smoothed skylight. This can be easily seen when loading with Optifine and a shaderpack. Lots of shader features uses skylight, for example, puddles in rainy weather. You can see there is no puddles near these blocks.
And for emitting opaque blocks with light level less than 14, their ambient occlusion seems also modify smoothed block light map. Smoothed block light of pixels near them can even drop to 1 (since this is smoothed light map in rendering, the light value in F3 screen is normal).

Using builtin/generated still cause some color change like [198,198,198] to [196,196,196], same as rotation (form -2.6° to -89°, the color keeps same). It seems that the new render engine makes the game can't restore the texture color in GUI.

Why not use the translation version on Crowdin? That version is pretty good and favored by we players.

If you guys just want models look like default in GUI, why not try rotating your model like this?
"gui":{
"rotation":[-60,0,0],
"translation":[0,0,0],
"scale":[1,2,0.01]
}
By rotate it 60° and rescale it, item models will be like using builtin/generated too.

They just need "cullface", but for some blocks like iron bars, adding cullface data will cause other bugs.

This is not a duplicate of MC-60242! This bug will even appear when using multipart in blockstates file!

Even Feather Falling can be applied on other pieces of armor. All protection enchantments do not mutually exclusive anymore.