I can't actually replicate this in my own data (I have no "0" page views in this scenario)
That said, perhaps it has something to do with the velocity being Page Views (Participation|Visit) / Visits
That participation attribution could be causing some oddities in the data....
That participation attribution may also explain why some values come out as infinity vs invalid... it's really rather annoying that we can't apply custom attribution to standard metrics to see what those values would look like... I suspect that in some cases the value is 0 (the same as Page Views) and in other cases, the custom attribution actually has a value that you can't see (and when you take a value and divide by 0 you get invalid - I guess 0/0 results in infinity rather than invalid??)
I can't really explain the null value... that is odd...
Since you are showing "Unspecified", I am going to assume that these represent your non-page view actions (clicks and such) that are getting values due to the attribution model.... You may want to filter out unspecified.
I don't use this metric much, and in theory, the participation attribution is what you need to see the average number of pages that come after the page (by row)... but I really don't know why you have strange results... As I said, it seems to be working for me....
Are you by any chance applying any segments to your panel? Perhaps something in the segment is causing the issues?? I have seen similar issues where a segment caused some of the values to return oddly (and once removed the data looked fine)
Basically, the segment could be filtering out the actual "page view" resulting in 0, but the participation of that page would still be included....