Per the card description for Grunn, the Lonely King:
When this creature enters the battlefield, it drains 5 mana from your hand then is reinforced.
Actual occurrence:
When this creature FIRST enters the battlefield, it drains 5 mana and then is reinforced.
When this creature arrives AFTER the first time, it only drains 5 mana but is NOT reinforced.
The second part, reinforcing itself for the turn if it is attacking and the lone creature, still works correctly.