You haven't given me enough details to make a good guess.
Are you using the software keyboard when this happens
software(onboard) keyboard
Are the comas randomly distributed, or are they always near the same words or letters?
random
One thing you can check is Settings > General > Keyboard > Text Replacement and make sure thre are no shortcuts defined that might cause the issue.
told you in ioriginal post that everything in 'settings' is unselected. don 't know how i could say it more clearly.
Oh, and are you sure they are comas and not periods? If the settings is on, a double space adds a period at the end of the word/sentence.
commas. settings selections are all set to OFF.