GUI Texture splitting is mismatching.
When I try to convert Texture packs to the 1.21 version most of the textures wont be recognize when playing Minecraft. this probably is happening because Minecraft added a new system of sorting textures like health or hunger bars. In versions before 1.21 texture packs had most of the time one PNG where everything was compacted like every heart ( poison heart , freeze heart , wither heart) and also xp bars and hunger bars. this changed in 1.21 where everything is now sorted in categories for each individual texture. I think the converter doesn't recognize this and just ignores it which causes the problem of texture not shown in the gui when using the converted texture pack.
old sorting of gui:
new sorting of gui:
Hmm, the texture gui splitting happened back in 1.20.2, it should definitely work if you are on 1.21 and the pack was converted from prior to 1.20.2
And the converter defenitly handles this because im the one who implemented it
Which release of the Converter were you using when this happened? This can help debug what could be the issue.
I had this issue as well... I am (somewhat desperately) trying to get this Pumpkin Party Remastered map and resource pack to work on 1.21 for my private homeschool co-op server as a Halloween event. I used the latest build from Jenkins [ResourcePackConverter-Gui-2.2.5.jar] and used the most recent version available for the map... But maybe I should have picked an earlier one? https://www.curseforge.com/minecraft/worlds/pumpkin-party-remastered/files - I got a successful conversion with a few "missing" textures, but when I go to load it fails to load the pack.