Yeah the 'he wants to go so he's going to play badly now' is a terrible excuse. I'll add Suarez to your examples - he was absolutely furious at Liverpool for not selling him, then went and put in what can be argued is the best individual season ever seen in the premier league.
Players who want moves don't suddenly become bad at football. They get their heads down and get the move later on.
....IF (and it is merely an if AFIIC) that's what Harry's up to then he really is lifting the lid on the true personality beneath the 'triffic lad' veneer.
I tend to think it's more likely it's just fucked with his head in the short-term. Seen it before with players like Eriksen..... Some players don't cope well with contractual uncertainty/disruption.