Morphine is commonly used as an analgesic to relieve pain in horses. It's completely within the rules except that it shouldn't be used on race days. So it seems likely that a simple error has occurred.
But it's not the first time that one of Lizzie's horses has tested positive. Moonlit Path was deliberately injected with tranexamic acid, upon the instructions of the trainer, in 2009.
I suppose the good thing about anyone who uses Agatha Christie's name is that we can all expect her to completely disappear, without explanation, for a while!
;-)