Why Facebook Graph API doesn't list all public events for a page?
There're lots of events for this venue:
https://www.facebook.com/pg/ClubRoxyPrague/events/
However when requesting with Graph API we only get a few. Here's link to Graph API explorer:
https://developers.facebook.com/tools/explorer?method=GET&path=ClubRoxyPrague%2Fevents&version=v2.12
Until recently this worked well for us. Maybe it's data access restrictions Facebook announced few days ago: https://newsroom.fb.com/news/2018/04/restricting-data-access/
Why the API returns only some events? Is there any way to fetch all public events hosted by a certain page?
Due to the breaking data access changes you mentioned, it already wasn't possible to access events directly (also see: Unsupported get request in get Facebook event).
It seems like your specific issue is a logical consequence of that decision (although the /{page-id}/events was still available up to today, I personally already found it pretty strange that events were still accessible indirectly).