Why? Just do a google search and read in the Spotify Community Forums. They are full of complaints.
But it happens on Sony, Samsung, LG and a lot of other devices.
The reason seems to be security issues.
I switch off my FiiO every night and boot it up every morning and never lost my offline content on FiiO.
But I have Spotify only on two devices, the FiiO X5 and on Samsung Galaxy Note 5.
There was a solution to that problem, that has worked with other users:
Remove Spotify from all your devices.
Remove all Spotify devices in your online Spotify account.
Install it on max. 3 devices ... you should be safe then.
I've got the impression, the issue has to do with some anti-misuse fuctions, that Spotify applies, when people use the offline feature.