Resource packs are bugging!

I have been downloading a lot of different resource packs lately, and I’ve run into a problem where some of the custom texture packs aren’t being used by the game and instead the default texture packs are used.

The issue is mainly with mobs, especially those added in the Minecraft 1.14 update. What’s even more strange is that the custom textures are utilized by the game for some of the texture packs, but ignored for some other texture packs.

I’m playing Minecraft on the latest version on Minecraft servers, without using OptiFine mod. My best guess is that abstaining from OptiFine might be messing with something, but I’m not at all certain what’s the issue.