Same problem here.
I had a working flow, worked fine for a month untill now..
Today I added 1 new field to the form and in the Sharepoint list.
then I wanted to update the FLow,
In the original Flow I cannot select the form anymore!??!!?
When I create a new Flow, I can not select formfields to store in SharePointlist.
Please let us know, what to do.
Or maybe we should not USE Forms in Flows?????