If you use custom models in a resource pack which use custom texture names that include special chars such as spaces or parentheses, and you try to add the resource pack to the selected resource packs list, it fails to do so.
But instead of just disabling just the affected pack, it seems like all selected resource packs get disabled, including default, resulting in a black/magenta square- texture for every single texture in the game, all models being 16x16x16 and no sounds at all.
F3+T re-enables default with the message "debug.prefix debug.reload_resourcepacks.message", indicating missing language-files as well.

An example resource pack is this one by Halbzwilling which is meant as a template to create custom items using predicate overrides: https://www.dropbox.com/s/loua16pv2olcvxq/XtraPack.zip?dl=1&v=THTB8MjOV24