Hi there Marcin,
Thanks for reaching out to us and sorry to hear about this issue. Let’s see what’s going on with your calendar!
First thing I would check is if the event is configured to be hidden in event listings. You can check that in the back-end of the site, when you are editing the event:

This option hides the event in all list based views but not in grid views (like month view).
If that isn’t the problem then, Can you please follow the steps described in our Testing for conflicts guide? This way we can see if this is a problem in our plugin or a conflict with the theme or other installed plugins.
Also, if you add more events to the calendar does the same thing happen?
Please let me know about this,
Best,
Nico