Not an expert but here's some ideas. Looks like they are in small pots so maybe transplant to larger with better soil. I think it's a coral bark maple, but maybe not. That's what the links are about. Good luck!

https://www.melindamyers.com/articles/edges-of-maple-leaves-turning-black

http://www.gardenality.com/Questions/2662/Plants/Trees/Coral-Bark-Maple-Tree-Trunk-Turning-Black-And-Dying.html

The dried black or brown leaf edges are an indication of irregular watering. This is somewhat common with JM's grown in containers that are allowed to dry out excessively between waterings. Soil/potting media should remain consistently moist but not wet.

Unless you have a very fast draining soil media in the containers you do not want them potted up in a too large pot. Most commercial potting soil can be overly moisture retentive and too much wet soil surrounding the roots can lead to root rots, which are most often fatal. Ideally you want just a 1" clearance surrounding the rootball when repotting. I'd be surprised if any required up potting now.

The issue discussed in the link about the coral bark maple is an entirely different problem dealing with a bacterial disease, Pseudomonas syringae. This is an early spring thing showing up typically after a harsh winter and is mostly limited to specific JM's with colored bark. It is NOT what's going on here as there is no evidence of the characteristic blackening and die back of the stems and trunk.

Thanks for both answers.

The soil used is an universal one:

https://www.brico.be/fr/tuin-buitenleven/planten-potten/potgrond/central-park-universele-potgrond-20-l/1002287

And the pot looks sufficient large for the size of the tree. I will try repot it to have a closer look at the roots.