Difference between previewing and editing a module
I created a module for the practical assignment of the developer's course. While previewing the module and the page itself with this module, everything works: I have 3 columns, they change depending on the width. But when I create and edit the page using the theme with this module, everything is reset to one column.
Difference between previewing and editing a module
@Sergey_N , most likely there's a css file attached to your theme that is overriding your display: flex rule on the "container" class. If you can send a link over though I'd be happy to take a look.