The bug

All user-facing strings are JSON text components, except custom_name in the potion_contents data component. This makes it impossible to give text formatting, colors, and use other text types than the hardcoded

{"translate":"item.<namespace>.<id>.effect.<value>"}

without using the custom_name data component. Using that has side effects like being able to be removed in anvils, and showing in item frames.

Effects

It's impossible to give a potion which grants effects with a custom name that cannot be removed in an anvil, nor shows in a map, that is:

• formatted
• colored
• different text types (text, keybind) (including resolved JSON text components of nbt, selector and score)
• a translation string of a different origin (such as using item.minecraft.stick)

The effects can also be resolved by making item_name used if neither custom_name nor potion are set, but that would not resolve the inconsistency of custom_name.