Bookable events are displayed erratically

Bookable events don’t display correctly.

First of all, when they show up, they show up as a duplicate.
Then you change the month to the right, then left and it disappears. You change month again and only one random event is being displayed…

WP cache is disabled. WP Rocket is disabled, WP and plugins are up to date.

Any idea why the events are showing up inconsistently?

Many thanks!