I like this project. One thing that'd be useful would be to make it reversible: i.e. if someone turns off the feature, can those comments still be seen in the vanilla mediawiki version of the talkpage. Currently when the beta is turned off, previous comments and the side panel disappear (example).
Topic on Talk:Talk pages project/Replying/Flow
I think you've posted this comment in the wrong place. Talk pages project/replying is about the "Discussion tools" beta feature, and the history of that example shows that you used the "Structured Discussions on user talk" beta feature, which is not the same thing.
@Trizek (WMF) might know what happened to your previous comments. I assume that it's something that's come up a lot at the French Wikipedia over the years.
It's perfectly clear what happened to the previous comments: they're at fr:User talk:Evolution and evolvability/Structured Discussions Archive 1. There's no actual bug being reported here, only the inherent incompatibility of Flow and Wikitext.
Aha, I see now! I'd just gone into preferences on fr.wp and ticked the talkpage beta feature without realising I was just activating flow on my user talkpage! Have actually tested with this the thing I wanted to test now and all worked as expected.