If you activate any filters (Campus, Day, Time, etc.) in the Group Finder Block settings, the page hides all groups initially, until you select a filter option and hit "Search"
In my opinion, the default behavior should be to show ALL groups initially, then use the filters to narrow the search. I understand that's only one philosophy to this issue, but the behavior does seem inconsistent with other Blocks that allow filtering. For example, the "Calendar Lava" and "Connection Opportunity Search" blocks don't require you to select a filter before displaying a list of results.
I'd like there to be an option to tailor the initial results displayed, before the user engages with the filters, or at least just a way to show all open groups when the page initially loads.
This is done and will be in v12.5.