Ran to another strange thing. Sarethi promoted me to councilman, and it updated the journal under "Great House Redoran" "For my services... promoted to councilman...". The I asked duties and he promoted me to Archmaster on the spot and journal says, "I defeated Bolvyn Venim and am now the Archmaster of House Redoran". But it didn't update the guest "Great House Redoran"?

It appears to be an oversight. The expected path is to be promoted through the topic 'advancement'. However, for some reason, Bethesda made it possible to be named archmaster if a qualifying Redoran asks for duties. While Ostar created such a response, he did not include the journal update for the 'quest' Great House Redoran. It does not affect anything, but if you want the update you may add it through the console:

Journal "JO_HR" 140

The matter is fixed for the next update.