I've noticed this problem before but didn't realise what I was doing. Just spotted it.
When I play an album in-page, via the player above the tracks, tracks start off scrobbling fine.
If I interrupt myself and stop the album, open another tab (in Firefox) to play something else, then return to the album later to continue, tracks stop scrobbling.
If I do a page refresh of the album, tracks start scrobbling again.
Don't know if this is a bug but at least I've realised what's happening. Thought I'd share.