Thomas. Thank you for bringing this issue to our attention.
The development team has logged it as an issue to be fixed. It won't make the 2.1 release but hopefully in a dot dot release if there is something before 2.2.
Just to clarify: we are using a different method of writing text into Pro PDFs that retains many of the advanced parameters such as intercharacter spacing in text placement. So we can't just switch back to the way that ArcMap did it without bringing back some existing issues in text placement in ArcMap exports. We are evaluating a way to resolve this that does not lose those properties, and we'll put it in when we have a solid fix.
As a workaround, we found that it is possible to select ALL the text in the string and then retype the new string. I know it's not ideal, but that should at least let you replace the strings or edit them as needed.
Thanks for the the workaround clarification. I would like to know if there are any updates to the PDF export engine allowing text edits in in the resulting document.
This is logged as BUG-000111662. We have it up for investigation in the release after 2.5 (which is already in progress), but be aware that the next release of Maps for Adobe Creative Cloud (which will allow you to bring Pro content directly into Adobe Illustrator for post-editing) will also allow similar workflows by going direct from Pro to AI.