An Ankarsrum issue/question - Potato Bread

I've been using my new Ankarsrum mixer without problems on the yeast breads that I normally make.  Recently I've experimented with potato flour.  In my first try I used one teaspoon per cup of flour (about 4%).  The formula uses 55% bread flour and 45% fresh milled whole wheat flour.  I also have melted butter and honey in the formula.  The hydration is about 67.6%.  I have made the formula without potato flour and all goes well.  With the potato flour I get a dough that seems heavy in the mixer and is really stuck to the bowl after kneading.  Once out of the bowl the dough feels soft and smooth. It rises well and when baked results in a dense crumb in a really soft bread.

Today I tried again and increased the potato flour to about 2 tsp per cup of flour (about 8%).  Same results.  I had to hold the mixer in place but the dough was quite sticky in the bowl when done.  I kneaded for about 15 minutes.  This was about 85 ounces of dough.  When I took it out to rise and hand kneaded it a bit I found it to be sticky and very soft.  It rose well.  It is now about to be baked.

So.  I get really nice bread but it really works the mixer.  Am I doing something wrong?  My non-potato formulas do not stick to the bowl like this nor are they are dense.

Mike