Vincent, what is your support case number and do we have your job logs already? I would like to ask devs to check this ASAP. If there is a bug, there is still a chance to fix it in the upcoming 4.1. I have not seen similar reports so far.
Ah, now when you elaborate the issue I understand what is confusing you.
Full backup indeed happens on Sunday for you. The full backup you are seeing for Saturday is the last syntethic full backup of previous incremental chain which started on previous Sunday. As you know, Veeam Backup produces synthetic full after each incremental cycle, which is why you see that full backup file for Saturday. It is expected for that VBK to have Saturday timestamp to, because it will no longer be touched by Veeam Backup: the periodic full backup on Sunday will start the new synthetic backup chain (new VBK).
Look for another, newer VBK file that was created during Sunday's full backup.