When there is a CSI and a rooted or query, the sub planning fails https://github.com/10gen/mongo/blob/master/src/mongo/db/exec/subplan.cpp/#L201 and we try to plan the whole query together https://github.com/10gen/mongo/blob/master/src/mongo/db/exec/subplan.cpp/#L127-L132. This fails when we try and build a classic executable tree as CSI is not supported in classic.