The way the hotbar and crosshair break is that the textures turn full white, except for the special spectate-icons in the hotbar, which turn full black. The crosshair uses the normal texture again when the hotbar is set to be shown.

How to reproduce

1. Spawn a mob that applies a shader when spectated. For this, it is possible to use creepers, endermen or spiders.
2. Switch to spectator mode.
3. Use the attack button on the mob created through step 1.
4. Make the hotbar or the crosshair visible.
   • To make the hotbar visible, just press one of the hotbar keys.
   • To make the crosshair visible, make the spectated entity look at a container such as a chest.
5. See that instead of the normal appearance, the hotbar and crosshair each use a full white texture.

Attachments

2020-07-14_03.01.52.png: The creeper shader in 1.16.1 with shown hotbar
2020-07-14_03.04.20.png: The same shader in 1.15.2, where the desired appearance can be seen