If more than one Identity is configured, "Reply All" on any received e-Mail with multiple "To:" and "CC:" addresses selects only the e-Mail address of the original e-Mail's sender. All the other recipients of the concerned e-Mail have to be entered manually. The KMail 5.3.0 (QtWebEngine) configuration is fresh -- neither imported nor upgraded from a previous KDE Plasma version. The local e-Mail directories are located in the directory: ~/.local/share/akonadi_maildir_resource_0/ There are 3 POP-3 receive and 3 SMTP transmit connections defined. If 2 Identities are deleted (the POP-3 and SMTP definitions remain; the Mail Folders defined for the other Identities remain as well), the "Reply All" behaves as expected. When one of the deleted Identities is re-defined, "Reply All" misbehaves.
Why multiple Identities? Take a look at "Create a new KDE Bugtracking System account": <https://bugs.kde.org/createaccount.cgi> PRIVACY NOTICE: KDE Bugtracking System is an open bug tracking system. Activity on most bugs, including email addresses, will be visible to registered users. We recommend using a secondary account or free web email service (such as Gmail, Yahoo, Hotmail, or similar) to avoid receiving spam at your primary email address.
The described behaviour only occurs when testing with multiple e-Mail identities configured in a single KMail2 client. If "Reply All" is executed on an incoming e-Mail with multiple recipients and, only one of those recipients is a member of the KMail2 client's identities then, "Replay All" works as expected. Possibly the explanation given by Laurent Montel to the Bug 377744 is also relevant to this Bug Report.
Given that this issue occurs only when testing with multiple identities on a single KMail2 client and, that with "normal" email traffic with multiple recipients this issue doesn't arise, please mark this Bug Report as being invalid and closed.