A folder belonging to a mail account should use the identity set for the account by default. At the moment, you have your typical mailbox with subfolders: user@example.org - Inbox - Drafts - Sent - Junk Changing "user@example.org"'s identity will NOT propagate this change to the subfolders at the moment. By the way, identities should match with the account by default. I don't see what's the benefit of having an "user@example.org" account with its identity set to "default identity" which is something completely different.
Perhaps related to https://bugs.kde.org/show_bug.cgi?id=310247
So after a little testing, it appears that identities are indeed inherited. Identity in settings dialog -> Identity of root folder -> Identity of other folders (Inbox, Draft...) This means that https://bugs.kde.org/show_bug.cgi?id=311143 would also be fixed already. It all looks very intransparent however, for example, why are identities of the account and the root folder not in sync and can be set to different things?
https://bugs.kde.org/show_bug.cgi?id=109966 looks to be the same as well
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This bug has been in NEEDSINFO status with no change for at least 30 days. The bug is now closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging Thank you for helping us make KDE software even better for everyone!