How to reproduce : 

On nightly, open project "Projet d'exemple public" and open calendar module. 

On 10 November 2023, edit "Présentation Rapport d'activités" recurrent event on "Finances" Calendar. 

Set beginning hour to 09:15 instead of 09:00

Click on save

Select this update for all events : 

=> Event on 10 November disappears. 

Note : if a refresh is performed, event reappear.