Evidence: with PNumber: Indexed=false, AutoIndexed=false over 5100 file(s), where PNumber holds the file number modulo 1000 and therefore about 1000 distinct values: collapsed HTTP 500 returned 0 row(s) against a RowLimit of 100, labels {"PNumber":[]}, grouping markers [], first row null; expanded HTTP 500 returned 0 row(s); the same query with no <GroupBy> returned HTTP 200 with 100 row(s); ViewXml <View><Query><GroupBy Collapse="TRUE"><FieldRef Name="PNumber"/></GroupBy></Query><ViewFields><FieldRef Name="FileLeafRef"/><FieldRef Name="PNumber"/></ViewFields><RowLimit>100</RowLimit></View>; collapsed body {"odata.error":{"code":"-2147024860, Microsoft.SharePoint.SPQueryThrottledException","message":{"lang":"en-US","value":"The attempted operation is prohibited because it exceeds the list view threshold."}}}. The throttle is in force on the grouped surface of this library, which is what makes the crux row above a comparison rather than a reading of a library that is not enforcing anything.