Properly set the drift time of foreign multipoles when rebuilding.

src/cell.c

@@ -177,6 +177,7 @@ int cell_pack(struct cell *restrict c, struct pcell *restrict pc) {
   pc->ti_end_max = c->ti_end_max;
   pc->ti_old_part = c->ti_old_part;
   pc->ti_old_gpart = c->ti_old_gpart;
+  pc->ti_old_multipole = c->ti_old_multipole;
   pc->count = c->count;
   pc->gcount = c->gcount;
   pc->scount = c->scount;
@@ -221,6 +222,7 @@ int cell_unpack(struct pcell *restrict pc, struct cell *restrict c,
   c->ti_end_max = pc->ti_end_max;
   c->ti_old_part = pc->ti_old_part;
   c->ti_old_gpart = pc->ti_old_gpart;
+  c->ti_old_multipole = pc->ti_old_multipole;
   c->count = pc->count;
   c->gcount = pc->gcount;
   c->scount = pc->scount;

src/cell.h

@@ -95,6 +95,9 @@ struct pcell {
   /*! Integer time of the last drift of the #gpart in this cell */
   integertime_t ti_old_gpart;
 
+  /*! Integer time of the last drift of the #multipole in this cell */
+  integertime_t ti_old_multipole;
+
   /*! Number of #part in this cell. */
   int count;
 

src/engine.c

@@ -4344,10 +4344,11 @@ void engine_do_drift_all_mapper(void *map_data, int num_elements,
 
       /* Drift all the g-particles */
       cell_drift_gpart(c, e, 1);
-
-      /* Drift the multipoles */
-      if (e->policy & engine_policy_self_gravity)
-        cell_drift_all_multipoles(c, e);
+    }
+      
+    /* Drift the multipoles */
+    if (e->policy & engine_policy_self_gravity) {
+      cell_drift_all_multipoles(c, e);
     }
   }
 }