I came across this last week, it's a known issue.
To droplist in 'Send Mail' transition action fails to show assigned username (defaults to first group) if 'Show full username' is enabled *2013 regression*
Customer recently upgraded from 2011 SP4 to 2013 SP3 and they are now unable to see any of the configured transition actions using 'send mail'. The droplist defaults to first group alphabetically. - Problem can be reproduced when using login names that differs from full name (i.e. login name 'IverothT' and full name 'Tor Iveroth') - Happens if user logged in to Administration tool has the setting "Show full user name" enabled in user settings (explorer tab). - The SQL query in S-055332 can help list all configured send mail transition actions.