Thanks for your swift response.
I trigger myself the conflict resolution thanks to two separate accounts (since it is my dev wiki) on two separate browsers (I tried a bunch, does not seem to be the cause of it).
I run MediaWiki 1.35.6 on a local EasyPHP dev server (Apache 2.4.25 x86 - PHP 7.4.19 x86 - MySQL 5.7.17 x86).
The error happens whether there is more than one paragraph or just one.
It seems that if I choose the newest, not saved yet, revision, the page reloads with a red "alert" background-color on conflicting paragraphs and deletes all choices.
If I choose the already saved revision, the paragraphs that had conflicts just disappear, but the page is still not saved: the interface shows then only the unchanged paragraphs, which is really weird.
I hear you about the PHP version, but every other extension works perfectly, so that would be odd.
Might be linked to my other extension configurations, I'll try on a fresh install and keep you posted.