Hey Erwan,
Thank you for reaching out to us!
To have the “All Day Event” checkbox ticked by default, you can modify the events-meta-box.php file on the-events-calendar/src/admin-views/
On line 143, simply add the word “checked” in the end of the input tag.
Unfortunately the template override method is not available for admin views, so you’ll have to re-do this customization after every update or explore alternatives to do the same thing from outside the core files.
Regarding the hours that are not disappearing when you check the “All Day Event” box, that’s odd! I’ve just ran a test in my own WordPress install and it is working as expected here. Can you please make sure that all Modern Tribe plugins are up to date? If all plugins are up-to-date and/or the update didn’t help you to solve this problem, I would suggest you to use Chrome Developer Tools to spot any console errors in your site. Access the page where you are experiencing this issue (using Google Chrome) and press F12. After that click on the Console menu and send us a screenshot / let us know if you see any errors in there.
Also, please share your system info with us so we can further investigate if necessary.
Best Regards,
Patricia