I am testing conversion from vBulletin 3.8.11 but BB Code does not seem to convert. Am I missing any settings or this is the normal behaviour?

Posts are rebuilt in the background after the conversion has completed.

@PurplePixel, in other words, after the conversion is complete, background tasks will run starting with the newest post and moving to the oldest post. Things will look weird until this is finished, but you can examine the newest posts at the time of conversion to see what the output looks like and make sure your BB code is being converted properly. You can see running background tasks from the ACP dashboard.

For our migration, we waited until a few months of posts had been processed before reopening the community to prevent confusion for members.