Typing on mobile is a very arduous and unpleasant process, so we should try keeping it to a minimum.
Sometimes your users will have made just a small error that can be fixed by replacing or adding a character or two (typically they typed something instead of @ in an email address etc.). Sometimes they will have made a big error and they need to retype the whole thing. So unless you have a way of knowing or making an educated guess as to which type of error will be more common with your users and your type of data - it seems like there's no way of knowing. However, small mistakes are much more common by nature.
Even if we say that we have no way to know this, deleting an entire field on mobile is still much easier for the user than entering an entire field - you just hold the Delete key. So if we keep the data, those who would've preferred that we clear the field have a small price to pay for our decision, since the field is deleted easily. But if we clear the data, those users who would've preferred that we keep it, have a high price to pay - they need to retype everything.
Seems to me that keeping the data is definitely the safer way to go.