Meeting and "Following tasks could not be completely planned"

Hello,

I have a problem with meetings. When I assign a resource to a meeting and compute an updated planning, I often get the message "Following tasks could not be completely planned" and I see that the resource doen't have enough free time for the meeting.
Indeed, there is planned work on another activity.

I thought meetings took precedence over other activities so I don't understand why the other activity is not rescheduled later.

The conflicting activity (planning mode is ASAP) is in the same project but not the same subproject.
When rescheduling, I tried to select the root project + all the subprojects + all the other projects where the resource is involved with no luck.
I tried to lower the priority of the conflicting activity but then the planned time goes to another activity, not the meeting.

We are using V6.5.2.
We will update to V6.5.6 and then I'll get back to you.
Thank you