Office 365 group available. This group was created over Teams and has an e-mail address firstname.lastname@example.org.
Since this group was created over Teams, it was added by an authorized member via Teams.
The mails sent to the problem group mail address do not reach the group members.
To solve the problem, there is an option called "subscribe new members" in the exchange group settings. When it is activated, the new added member is sent by mail. Or when a user has been removed and added.
But when we consider that there are more than 50 groups, we think that this is not a very healthy and logical method.