Since the painting changes going from 1.13->1.14 custom paintings do not alow non exicsting pixels in them, this is a huge and very annoying problem when it comes to resorce packes that used these tex MCME, where this was used to makes som really sick things like this door;
something thats not possible anymore and instead look like this:
the same gose for all other paintings all over the world, as just anouther example here is a sword, one of the most common paintings:
it just look really dumb and it really hinders creativity somwthing this game should excel at. It should not be that problematic to fix due to the fact that this worked before 1.14
This is marked as a bug, so Mojang will/may eventually fix it (unfortunately, they can be quite lax on fixing non-critical bugs, for example, I recently fixed a nearly 8 year old bug which was extremely easy to fix; Looting already requires that the killing blow is directly traceable to a player so I just made it require that a player is the immediate source):
Otherwise, this is probably a side-effect of a change first made in 1.8 so blocks that aren't supposed to have transparent textures no longer do to patch x-ray texture packs, though the addition of custom block models enabled far more powerful x-ray texture packs, even enabling you to see all ores, not just ones exposed in caves (like in Spectator mode), so this seems like a pointless change to me and they may as well just remove the code that removes transparency (I presume it sets the alpha of all pixels to fully opaque unless the block itself says it supports transparency).
Do you think it's possible to create a forge mod which fixes this bug?
Absolutely; mods can do literally anything and I've fixed dozens of bugs myself, although I do not have the capability to make such a mod, even for 1.6.4 (I only do non-Forge mods and don't take mod requests); I have no idea where the code might be or what it looks like but it would almost certainly be in the texture loader code (1.6.4 does have code that removes transparency from parts of the player skin that aren't supposed to be transparent).