The Bug

When you use resource packs with custom texture in folder assets\minecraft\textures\gui,{{ }}the texture will lose its Opacity map channel and would be rather completely transparent or untransparent. WHICH WAS NOT A ISSUE IN LOWER VERSIONS.

It's not a big issue that breaks game balance or makes the game unplayable, but the fact that it could break some custom resource packs should be considered.

How To Reproduce

1.download the resource pack provided in the attachment below

2.apply the pack

3.open up player inventory in creative mode in MINECRAFT 1.14.4

you will see something like this:

4:open up player inventory in creative mode in MINECRAFT 1.15 snapshot

++**you will see something like this:

Some More Pictures To Show The Difference

I hope that would demonstrate things clear enough

Conclusion

Since rendering the player inventory opacity is very unlikely to cause severe impact to framerate or any other rendering problems, I do not think this behavior is intended. I would appreciate it if the bug can be fixed in the 1.15 final release. 

HUGE THANKS FROM TEXTUREPACK COMMUNITY

by the way player armour opacity is also not rendered properly