Custom textured glass (with transparent color on it) appears as a fully transparent block

Pretty much works as intended because some block don't support part transparencies.

If this isn't a bug I'm sad 😞

I know, I wish they added the support, or I might be completely wrong and it is a bug.

Since transparency support was added, it would make sense to provide it for any block. It does appear that each block has this manually added - We have so many bugs with transparent blocks not appearing transparent in various views (in-hand, third-person in-hand, being pushed by pistons, etc.) and each seems to need attributes manually applied to function correctly - so that would be a task if this can't be universally applied.

That, or it's intensive just to toggle support, and they don't want to bulk up the game with checking for transparency and such... In any case, once mod makers get a handle on this version, they'll probably find some good solutions which the developers can take example from. 🙂

Cannot confirm, custom textures for glass are working as designed. Please check your resource pack.

Can confirm, check this issue: MC-35245