I noticed in a modpack I was making for 1.12.2 (Forge Ver 14.23.5.2847), that after activating the Spellbook GUI, it wasn't showing on the screen. I thought it might be a HUD problem, so I created a new instance with only Angel of Vengeance, and the RPG-Hud mod. Sure enough, with the custom HUD disabled, AoV rendered the Spell hotbar fine, but as soon as I switched up the HUD to a custom one, no matter what combination of settings I tried I wasn't able to make the AoV Spellbar show up.
I tried setting it to render over the vanilla hotbar, I tried changing the RPG-Hud settings to allow modded renders, and I increased the Starting Stencil Buffer ref Value slowly to the max, but it didn't seem to help.
Here it is with the RPG-Hud turned off

And here it is with the exact same settings, just turning the RPG-Hud on.

If there are any workarounds for this, please let me know as I would love to use both of these together. This is a truly amazing mod. Thank you!