Evidence: Id is natively indexed on every list and library (library-index-threshold-probe.js, 2026-09-08), and no write in this run touched it: collapsed HTTP 500 returned 0 row(s) against a RowLimit of 100, labels {"ID":[]}, 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="ID"/></GroupBy></Query><ViewFields><FieldRef Name="FileLeafRef"/><FieldRef Name="PChoice"/></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."}}}. #480 measured the same refusal on the other fixture. A native index does not lift a group-by either, on either library, which is the reading the crux row above sits beside.