I think your fermentation temps are the likely culprit. If you're able, it'd be interesting to re-do the brew and pitch at 45, let it rise naturally to 55, then do a rest at 70 once the fermentation is almost complete.
Still, a 25 isn't too bad and if you enjoy it, so much the better.
One last thing, how much yeast did you use? Lager yeast at low temps tend to need more cells for clean fermentation. If you're able, make a big starter!