Why calling an async method gets its body executed sometimes but sometimes not?

Question

I am calling a drop method: It gets called, calls the api alos but sometimes, doesn't update the database even though it hits the line

   db.Entry(i).State = EntityState.Modified;

but still doesn't update. It's been a day trying to figure it but can't but when I make the function SYNC instead of ASYNC in the API then it never happens, why?

 drop(event: CdkDragDrop<string[]>) {
      const group = event.item.data[0];
      const items = event.item.data[1];

      moveItemInArray(items, event.previousIndex, event.currentIndex);
      this.changeDetectorRef.detectChanges();

      setTimeout(async () => {
        await this.newSvc.updateDo(items.map(i => i.Id))
      });
    }

newSvc:

  updateDo(items) {
    let url = `${this.URL}updateDo`;
    return this.http.post(url, items).toPromise();
  }

Api:

[Route("updateDo")]
[HttpPost]
public async Task<IHttpActionResult> updateDo([FromBody] List<int> args)
{
    int dpO = 0;
    foreach (var a in args)
    {
        dpO++;
        var i = db.NewSs.SingleOrDefault(n => n.Id == a);
        i.DO = dpO;

        db.Entry(item).State = EntityState.Modified;
    }

    await db.SaveChangesAsync();
    return Ok();
}