Is there a new issue or is it still related to the previously reported issues ? Unfortunately, without a change in Flarum Core, I don't see any easy solution.
The issue relates to converting a post into text and there's the one reported in FriendsOfFlarum/follow-tags7 and the other linked by David flarum/core2105
As previously discussed, I have not been able to find any issue with pretty-mail. The identified issue relates to FoF Follow-tags and FoF Subscribed, but ultimately it's a missing feature in core that creates limitations here.
A workaround I could implement is to have the first post be an actual post and not an event post, but this means loosing the custom styling, the custom link, and the excerpt "read more" button. This also prevents using features I plan to add in the future, including showing the Wordpress post author.