AM/PM is cut off when using double-digit times in the Date/Time Control in Responsive Forms. This is a known issue in Nintex Forms 2013 18.104.22.168 and Nintex Forms 2016 22.214.171.124. This is strictly visual and has no functional impact on Responsive Forms.
Currently, the following font sizes produce these results: 12 - Fits properly. 14 - Begin to see 'M' be hidden. 16 and 18 - Hides half or most of the 'M'.