Clement,
Thank you for reaching out and welcome 🙂
I did a quick check to see if I could reproduce the behavior using Event Tickets (RSVP) and was unable to do so (see screenshots below).


I also attempted to modify the ticket start date and was able to successfully do so (see screenshot below).

Can you please confirm you’ve reviewed our Testing for conflicts guide to see if you can reproduce the issue? Let me know and we’ll continue investigating from there.
Thanks again and have a great afternoon. Cheers!