Baby Potatoes are slightly bitter

I have been growing Nicola (sec early) spuds this year for the first time and I keep noticing that they are quite bitter and I am wondering what could be causing this?
Is it variety? When I harvest them I store them in paper bags and then put them in the cupboard, could this be the cause?

I grow them in Sylvagrow compost. I harvest them after 70 days (As they are pretty big at this stage) could harvesting them too early cause it?

None of them have any green on them at all. They rarely taste creamy, mostly they are a bitter slightly burning aftertaste


Edit: Some other factors I should add in are, I harvested some early Jul and was still eating them by Early August, could this be the issue?

They are also grown in a very hot spot in my garden

 

Hi would say also growing conditions and storage also seen this
How Do You Stop Potatoes From Being Bitter?
If you’re looking to stop potatoes from being bitter, here are a few tips:

1. Start by boiling the potatoes for about 15 minutes before peeled and cutting into small pieces.
2. Add salt and fresh lemon juice to the water before boiling, and then simmer for about 20 minutes until the potatoes are cooked through.
3. Remove the potatoes from the water with a slotted spoon and let them cool slightly before slicing into thin pieces.
4. Serve the potato slices with some butter or olive oil on top and some chopped fresh parsley or chives if desired.