(I would have assigned that to a later version but there only is 3.2.0-a1 currently, this needs to be done before release, however)
The current situation is, that there is a command for the CLI to reparse all posts but some shared hosters don't provide shell access so this is pretty impractical.
Old posts can still be displayed since the old BBCode parser is left intact, however this should be removed within the next two versions. (afaik?)
That's why we need a cron job for reparsing posts.
There is a topic on area51 which might be helpful for this but I'll especially highlight these two posts: https://area51.phpbb.com/phpBB/viewtopic.php?style=2&p=281566#p281566