Here is an image of the drooping branches

Not an expert myself but have read on another forum of people experiencing black markings on this particular maple.The cause is yet unknown and fungicides etc. don't seem to help.However it doesn't seem to affect the plant greatly as new growth is usually ok,just looks unsightly at certain times of the year.As to the drooping,perhaps settling in after replanting?..maybe check it's getting watered ok.

There are various reasons JM's develop leaf spots and some have nothing to do with diseases. With a very young tree just recently purchased from a nursery, I would suspect physiological leaf spot or a reaction to envirnomental factors - weather, mechanical damage, jostling by customers or due to travel, etc. Certainly nothing I would be overly concerned about at this point. Just keep an eye on it so you will know if the situation starts to escalate. Just an FYI but one wants to be extremely hesitant about spraying JM's with anything other than water. Their delicate foliage reacts badly with most pesticides and creates a phytotoxic reaction that is often worse than the problem being treated.

Drooping condition most likely caused by planting/transplanting, which can cause temporary shocking. Keep out of sun and properly watered until it recovers.

FWIW, I am not aware of 'Akane' having any particular issues with leaf spot. I grow it myself and it is a pretty trouble free cultivar IMO.