Tom, thanks for sharing this issue. Unfortunately it is not something we can address since it is an issue with the Discourse forum software.
If you are interested in pursuing this, you can have more impact and get a better response in the Discourse forum. You might start with this topic, where other users have reported other issues with the Details HTML tag:
https://meta.discourse.org/t/details-tags-munged/40823
When I have some time, I'll do some testing to see if I can gain any insight into this issue.