Unexpected Missingno./Rhydon nickname details

Master Ball:
If you choose "No" to nicknaming "MASTER BALL" or "PP UP", it will be given that nickname anyway.

If you choose "Yes" to nicknaming "MASTER BALL", the nickname field will appear blank initially and if you choose to immediately end nicknaming the Pokémon, it will be named "RHYDON" properly terminated by five 50 bytes.

The nickname field automatically fills in with "MASTER BALL" (with the second "L" going off the screen) if you press A on a character.

If you choose to end the nickname after this, or empty the name field, you will receive a Rhydon with the same name ("MASTER BALL") as the nickname if you chose not to nickname it.

If you don't remove all of the characters, then you can receive a Rhydon with the nickname you choose and it is properly terminated.

PP Up:
The behaviour on the nickname screen works the same as "Master Ball", except that if you press A to print "PP UP" on the nickname field and then end the name, its internal name will be "PP UP[50][50]LL"[50][50]", rather than "PP Up[50]POKé " if you chose "No" or blanked the nickname after having it print on the nickname field.