Was the money in the Roth 403(b) the result of an In-plan Roth Rollover within 5 years of 2014 (2010 through 2014)? If so, that would be in box 10 and in your Roth IRA would have become basis in Roth conversions before 2016, essentially equivalent to Roth conversions but would be present on Form 8606 line 24 instead. It's not possible that your distribution from the Roth 403(b) didn't include some basis that would reduce the taxable amount of your Roth IRA distribution in 2020. You might need to go back through your Roth 403(b) statements to find your basis in Roth contributions and In-plan Roth Rollovers if the plan failed to reflect these on the 2014 Form 1099-R.
