Advantage is granted because they can't see you so you always have surprise against them. It's more roleplay than strict rule. They have disadvantage against you because they are not in the bubble, but they know the general area it exists in, they could throw something in and just hope it hits.
Even with reduced numbers Crolwell straight murdered the frogman and destroyed the dagger. He specifically mentioned coming down on where the dagger was stuck into the creature. Make sure you take a dagger from your inventory Ütés. :P
Advantage and Disadvantage can be given situationally if it makes sense. So I sort of house ruled it, just forgot to mention it right at the start of the battle. This works for them and you, so if you leave, and they get into the bubble you would be at disadvantage against them until you were either in there with them, or they came out at you.
Once you leave though you no longer surprise them, so running out to do melee attacks against them gets you seen again. If you are an archer or ranged attacker, this is your dream haven. :)