The only change I see to sudo between 5.3 and 5.4 that might be related is this one. You could try backing it out via "patch -R" and see if the old behavior is restored.
- todd Index: sudo.c =================================================================== RCS file: /home/cvs/openbsd/src/usr.bin/sudo/sudo.c,v retrieving revision 1.43 diff -u -r1.43 sudo.c --- sudo.c 8 Jul 2010 21:11:31 -0000 1.43 +++ sudo.c 6 Nov 2013 20:14:47 -0000 @@ -305,7 +305,7 @@ log_error(NO_STDERR|NO_EXIT, "problem with defaults entries"); /* Set login class if applicable. */ - set_loginclass(sudo_user.pw); + set_loginclass(runas_pw ? runas_pw : sudo_user.pw); /* Update initial shell now that runas is set. */ if (ISSET(sudo_mode, MODE_LOGIN_SHELL))