The bug is if you give a potion effect level higher than 4 , no level will be shown in inventory as it is level 1.

This is an old issue but not problematic previously so I haven't consider it as a bug. You can not obtain level 5 potion effects before unless commands or external tools . So it make sense to make stuff that not suppose to happen don't show up .

However , in the 13w24b enchanted golden apple will give out regeneration 5. Which this issue become annoying . Recently there are a lots of contet updating to be harden the difficulty . When I first try the snapshot 13w25c ( before that I am just playing 1.5.2) , I think that apple has been changed into level regeneration 1 only! This is misleading .
What you expected to happen

After Eaten a enchanted golden apple , the inventory shows up regeneration V

What actually happens

After eaten a enchanted golden apple , the inventory show up regeneration .
So I suggest at least show the number up to 10, like enchantments do.

Recreated bug .
1.get a enchanted golden apple in creative
2. Turn into survival
3. Take some damage
4. Eat the olden apple .
It will recover so fast that nearly instancely recover your health . However , according to the inventory , it is only regeneration 1! Does it make sense ?

Or another way
Use / effect (your name )( any code ) ( any time ) 4

The inventory will only show it is only level one .