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

1. Eat a golden block apple (the glowing variety) while in Survival mode
2. Check your inventory screen. It will say you have "Regeneration"
3. 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.