Emails are not moved but duplicated when using an Exchange account

I can reproduce the following behaviour when using emClient on my MacBook and on my iPad in parallel:

  1. I sent an email to my Exchange account, and I can see it in the inbox.
  2. I move the email on the MacBook from the inbox to a folder. This works, and the email is no longer in the inbox. I can see the effect on both devices.
  3. I move the email on the MacBook from one folder to another folder.
    ==> On the MacBook, the email is moved, and everything is good.
    ==> On the iPad, the email is copied (and not moved), and I can see the email twice in both folders.

The problem persists after syncing. This is extremely annoying, because I have to delete all duplicates manually.