Users with full access to someone else's mailbox can no longer send emails

BMERCIER45 Here's the article that tells you the truth: https://support.microsoft.com/en-us/topic/how-to-grant-exchange-and-outlook-mailbox-permissions-in-office-365-dedicated-bac01b2c-08ff-2eac-e1c8-6dd01cf77287

In particular, this is the paragraph to pay attention to:
Full access permissions let a user directly sign in to a mailbox by using Outlook or Outlook Web Access (OWA) and then add that mailbox as a secondary mailbox in Outlook. However, full access permissions do not let the user send as or send on behalf of the mailbox. Those permissions are granted separately.

I'm not completely sure where your screenshot has been taken but the permission description doesn't seem to be correct. Here's what you'd see under mailbox delegation in EAC. You see, sending permissions are separate from Read&Manage.