Versions Tested: Minecraft 1.18.2, Stylish Effects 3.1.0, Ars Nouveau 2.6.0
Forge Version: 40.1.52
Checked for interference in a group and alone. Potion effects don't appear with the Stylish Effects mod but appear once Ars is added.

Photo of effects appearing twice

Should be resolved in v4.0.0 for 1.19. Please reopen if it's not.

This issue still exists. (Minecraft 1.19.2, Forge 43.2.3, Stylish Effects 4.3.1, Ars Nouveau 3.6.2)
With Ars Nouveau installed the GUI renderer does not show (inventory renderer is fine). Remove Ars Nouveau and it appears. I suspect this is a conflict with Ars Nouveau's mana bar overlay, which can't be disabled.