I would need more information to tell you what’s going on, but there is a bug in macOS / iOS that affect Nitro with masks. Specifically, if you have the same adjustment (e.g. Curves) in the main interface AND you also brush Curves, then the system gets confused and it applies to the whole image.
To troubleshoot this, take a simple JPEG, make no edits and then apply a mask with Curves. Do you see the same problem?