The plastic sack is responsible. The spuds had limited access to nutrients while in development. I would grow them in the ground with plenty of compost.

This has happened to us too when we've grown in sacks - there's no flavour. This year we grew some in an allotment and harvested our first new potatoes yesterday. They tasted just like they used to! Delicious.

I would say there are several reasons why your potatoes are tasteless. Firstly the variety of potato makes a difference. Then, they need to be fed, if possible with an organic potato feed but as they demand lots of water then the nutrients are quickly washed away. Finally, yes it's better to grow them in the ground but they do tend to take up lots of space, which if limited would be better used to grow other vegetables.

Lack of taste is down to variety and growing medium. It's nothing at all to do with the plastic sack.