Some parts of the durability bar are at the top of the actionbar text, while other parts are at the bottom of the text. Also affects the capacity bar of Bundle.
In the attachment, there are two translucent characters with negative ascent covering the hotbar (position 0 and 1). Some parts of the bars are darker because these parts are covered by the characters.
Update: the covered parts are transparent parts of the item texture.
Can confirm.
If a part of durability bar doesn't cover the item, it also won't cover the text of the action bar.
It is because the depth test is disabled so that the durability bars simply overlay on the screen. The source code is in
renderGuiItemDecorationsinnet.minecraft.client.renderer.entity.ItemRendererThe expected behavior is to enable depth test when rendering the durability bars.