I right click an account parent folder / folder properties / Unckeck => sync when selecting this folder / Check => Use parent options. Yet every time I launch kmail 1) all folderssync 2) these options seems to reset.

Any ideas?

Kmail - folder properties keeps resetting
You want to store mails in a special folder. ... Again there are options, but one of the best known is procmail. ... This one is mostly a matter of restarting so KMail can fetch those pesky items. ... Now that address keeps popping up as a suggestion, every time you try to send to the correct address website

This mainly because some folders are categorized System folders (special folders). If the designated folder is not marked as a special folder, then KMail may not work properly. The application ends up recreating or resetting the folder. I think this may help you out
https://docs.kde.org/stable5/en/pim/kma ... -over.html