The 1st import works fine. Everything gets imported and I skipped nothing.
Then sometime later, I go to do a second import to pick anything new from the live vB forum. I check 'Retain imported content IDs', put in all the config data, click validate, and before it gets to the page with all the import steps it says "Processing... Deleting... Threads" which takes some time.
(Nothing had been deleted from the vB forum between the 1st and 2nd import.)
Then it goes to the page with the import steps. Then I load the archived import log and proceed through all the steps.
The result is that while new posts have been imported several forums have been deleted that were present after the 1st import.
The deleted forums where id=1 in vB which was a Category and all it's Child forums.
Why are some forums deleted?
Should I not check the 'Retain imported content IDs' on subsequent imports?