I'd issues scrobbling too with Poweramp v3 alpha.
Now I found a solution and it works fine since 3 hours using SLS, my settings are:
scrobble via Last.fm app enabled
scrobble via Simple Last.fm disabled (yeah, sounds weird but it really works)
active Apps: Poweramp checked
scrobble after: 50%
now playing: chedked
when to scrobble: after each song
scrobble on playlist end: checked
Note that newer android versions have battery saving features and kill apps if they're not on the whitelist as soon as the screen is disabled, so you need to put SLS on the whitelist.
If I pause a song it isn't scrobbled directly, but it will be scrobbled as soon as I continue listening or start listening to new songs, if the 50% were already reached)