I was finially in a position to test that issue with my Part 2 party.After talking to all five Lennies, my Doom Knight's XP was 17,295,997. After talking to Lennie the Elder and opening the portal to the Vally of Stones, his XP jumped to 18,795,997, a 1.5M XP gain and NO Reset occurred even though his XP before the bonus exceeded the 16,777,215 "limit".
I dunno what is happening here. GrayFace indicates he DID NOT address that issue in his patch and I am sure MOK did not either. It is obvioisly NOT an MM6 issue and it appear not to be a TCC one either (at least in my installattion).
Hopefully BDJ will check in one of these days and we can ask him to check it out.