I couldn't find an existing post about this, but my guess is if there was one, it's been closed as "Invalid". However, the bug now occurs in pure vanilla survival, without cheats.
Steps to Reproduce
Eat a golden block apple (the glowing variety) while in Survival mode
Check your inventory screen. It will say you have "Regeneration"
Observe that you actually have Regeneration V - NBT editing will confirm that you have Regeneration with Amplifier 4 (an amplifier of 4 means level 5), and simple observation of your healing rate will show that you definitely can't have ordinary Regeneration, so the status text in the inventory is wrong.
Needless to say, it's impractical to come up with a way to display every conceivable roman numeral (or even just those up to 128, the max potion effect level), but enchantments display the correct value up to 10 - potion effects should likely do the same, especially now that you don't need to cheat to get effects higher than level 4.
Related issues
is duplicated by
Comments


There is no potion.potency.4
at crowdin.net

I've noticed the game does not even bother to render "potion.potency.4" on the inventory screen, even though it will render the equivalent for enchantments above 10 on the item tooltip, and even renders the "potion.potency.4" for custom potion tooltips.

Confirmed for 14w26c

Confirmed for 1.8.8 and 15w44b.

Golden apple no longer gives regen V, resolving to MC-10755 as it's an unsupported amplifier.
Confirmed.
Steps to reproduce:
Issue
/effect <name> 21 50 3
- Shown isHealth Boost IV
Issue
/effect <name> 21 50 4
- Shown isHealth Boost