Portals aren't meant to be printed - even FMP itself has big trouble doing that (FMI specifically do not advise to print portals), never mind IWP/browser.
To solve this, you'll need to create a separate layout that shows those related records and print from there.
Hi Dirk, thanks for the info.
Sorry, but that doesn't really fit with this problem. The layout prints perfectly from FileMaker Pro including portals.
The layout renders perfectly through IWP, but just goes wrong when printed. As far as I can guess, the problem must be down to the HTML IWP is outputting (with lots of inline CSS styles), which points to a bug in the IWP rendering.
I can provide PDF prints from FMP / IWP for comparison (via a private message).