I've not read all the comments so I don't know if this has been touched on already, but I saw this and I was quite shocked:
"During fight week, Joshua told Carl Frampton that after beating Wladimir Klitschko in a topsy-turvy 2017 bout, he had informed trainer Rob McCracken he would "give up" if he needed to be in fights which played out like a rollercoaster again."
https://www.bbc.co.uk/sport/boxing/48489862
You can't have that mindset in competitive sports otherwise you may as well give up now. He's basically saying if it gets too tough, he'd rather get beat, instead of proving why he should be the best.
On the flipside, he has said before that he doesn't need money, has secured enough for him and his family for the rest of their lives and is only boxing for personal glory. He said this 5 months ago in an interview with IFLTV. He's been ranked #1 and beaten a lot of big names, whilst also fulfilling a childhood dream of being world champion.
Maybe he's just lost motivation for the time-being?
We saw it with Fury too.