Cause of issue is likely to be linked to MC-269083 - would recommend looking into this as well.
The bug
If the experience bar or jump bar is given a texture with translucent pixels, they will be rendered completely opaque in-game. This is in stark contrast to boss bars, as well as just about every other HUD texture - all support translucent pixels.
As of 1.20.5's development, some more UI textures render as opaque, however this results from a bug (MC-268171). As that bug is planned to be fixed, this case should likely be fixed as well, hopefully alongside other UI translucency problems (MC-165182, MC-174732, MC-200987, MC-251303, MC-269083).
How to reproduce
Download and apply the attached resource pack
Look at the experience bar
Summon a wither and look at its boss bar
Expected results
The boss bar would be translucent, and the experience bar would also be translucent.
Actual results
The boss bar is indeed translucent. However, the experience bar is completely opaque.
Linked issues
relates to
Attachments
Comments
[Mod] Jingy
muzikbike
Since that ticket mentions many different texture files I'd say it's probably a better idea for each thing to get its own ticket so that we can see what's intended and what's not, especially since the XP bar's behaviour clashes both with all other bars in the game and all other GUI elements.
[Mod] Jingy
That's fair. I can confirm the issue.
eowyn36
The XP's transparency is described in MC-109447, which was resolved as Won't Fix. Not sure if it's a duplicate or not, but I figured I'd mention it.