xform: fix `partition_by` would sometimes return dangling reference
xform: fix `partition_by` would sometimes return dangling reference

Valgrind, you are awesome!

AbletonAG/atria,AbletonAG/atria,dodheim/atria,Ableton/atria,Ableton/atria,dodheim/atria,AbletonAG/atria,dodheim/atria,Ableton/atria