Links set to open in new window in 3.0.12 show malformed on edit in 3.1.2

Auto-detected links that were set to open in a new window using the core mod explained in this knowledge base article: (https://www.phpbb.com/support/docs/en/3.0/kb/article/links-opening-new-windows/) on a 3.0.12 forum are malformed when viewed on a migrated 3.1.2 forum.

I recently migrated my forum from 3.0.12 to 3.1.2. When looking at the database for a post made in the 3.0.12 era, I find this code for an autodetected link:

<!-- m --><a class="postlink" href="http://www.smws.nl/" onclick="window.open(this.href);return false;">http://www.smws.nl/</a><!-- m -->

When I open that post for editing, this is visible in the editing textarea:

http://www.smws.nl/" onclick="window.open(this.href);return false;

Which then gets saved in the database on saving the post, not breaking the link, but adding the onclick code as visible text to the post.