Betfair have confirmed that it is normal behaviour for there to be a LTP visible for non-runners in the streaming API. Please see below the statement about this that I received from Betfair:
"LTP is lost to the polling API after runner removal as it no longer has the matched bets to inspect to see the last matched and determine its price. LTP isn’t lost to the streaming API as the data have already been published to the stream and we didn’t (and I can’t think of any reason why we would) explicitly make a point of erasing it."