Y888099 wrote: Simple "code" tag.
Since nobody else here has been able to duplicate your issue, I suggest that you go to the phpBB website to see if anyone has filed a bug report that matches your issue. https://tracker.phpbb.com
If there are no such reports, then consider filing your own bug report, providing detailed information about your local posting environment and the explicit steps you take when you experience your problem.
For example, here is a report that seems similar to your issue, but it appears when using the "php" qualifier in the "code" tag and does not appear when the qualifier is not used. https://tracker.phpbb.com/browse/PHPBB3-9105