Your Ninja is fine and doing potatoes exactly as they would in a more conventional form of cooking if re-heating.
I tend to use sweet potatoes now, hard to peel (bit like turnip) but cook and freeze very well.
You could also treat your stew a bit like Scouse when preparing. In Scouse the potatoes are cooked for that long they disappear and become the starch that thickens the Scouse. When you hear someone say 'you could stand a spoon up in me Ma's Scouse' it's the amount of potatoes used. There is a codicil to that though but I realise I've gone on a bit, I get carried away when talking food prep, sorry Norma.
To sum up Norm, potatoes go hard and watery in a domestic freezer. A few exceptions, mashed with loads of butter, cut very small and frozen in a stew, chipped and cooked (not great but acceptable)
and baked with cheese and butter on top.