I have recently started having issues when grouping a SharePoint list which I have a filter on, If I filter the list for "not equal to" and leave the box blank (to show all items that contain something in this field, then group by this field, it does not display any results. As soon as I remove the grouping it displays the appropriate results. This worked previously but has stopped functioning appropriately in the last few weeks.
Hi Steve I know this post is old but I too am recently experiencing same issue and what I think is the problem is that only recently the list has exceeded 5000 items. My client just wants to create a view based on a date field containing a value, if its blank then they don't want anything in the view. Maybe not possible or there might be better way to obtain the same result.. I am going to ask myself too.
Maybe check your list anyway and see if its due to 5000 limit (or I believe if you are coming close to that limit the same constraint would apply). Cheers Mark