I can't comment so I'm putting this as answer.

Can you check whether the user is being created successfully with this line?

var result = await _userManager.CreateAsync(user, password); 

I’ve seen a similar issue before in a Django app. We were also able to create users, but they couldn’t log in — we kept getting “invalid login.” The problem turned out to be that we were saving the password manually (even with hashing), but Django’s login system compares the input password using its own internal hash algorithm. Once we switched to using Django's built-in password hashing at the time of user creation, login started working fine.

I think there is a highly chance that you are not providing the right password ( the one associated with the user generated by Identity).