Diamond+ LP Busted?

Should I be getting 13-15LP and losing 20+ when I have 50% win-rate in Diamond?

If not, why is this even happening? There is absolutely no reason for me to be getting 13-15LP when I have 50% win-rate, right? Then why? I have to win 4 times in a row to recover from 2 losses, it's outrageous, what's the point of solo q if it works like total crap.