Fat Banana

Mmm it might be that the soild is too acid or even the PH from your water, not sure if you checked that, but the strange is that its only 1 plant of the 6, in my opinion just let it grow, continue to provide slowly water when there is not too much heat outside and you will see her recover and grow. watering in the middle of the day with lot of heat might also burn the roots

Look for a spot where she receives some hours of sunlight but indeed not too much as they are still quite young, it also depends on the temperature outside, i have had small plants always outside with around 26 c° and never nothing happened, just be sure to water in the early morning and maybe use airpots, fabric or white pots as its better that the typical old black pots. Putting them inside outside inside i dont think its needed, thats normally needed when the plant was grown indoor the first couple of weeks and we want to put her outdoors, than some people do this to make sure the UV and direct sunlight exposure in gener doesnt burn the plant