We have a list of categories (see screen).

When adding a new event on the desktop size screen, we can't select any category. At first we do see 3 categories, but once you click them they disappear.

On a mobile size screen, we can select each category without a problem. However, we still see only three categories (first three).

Editing in the backend goes perfectly fine.

What goes wrong?

JEvents: 3.4.50 Stable
Joomla! 3.9.12
Gantry 5.4.31