Hello,
I discovered this bug when using a roundcube plugin for qmailadmin that sets the vacation message.
When it saves the message it uses the moduser form which brings fsaved state, and there's no value in the checkbox. Therefore when it is saved it doesn't write the needed value for fsaved, which is "on". Then, if fsaved was originally checked, the plugin sends the checked input with an empty value. If you look at user.c it compares the value with the string "on" to set the "Save a copy" forward option. Finally, the not desired side effect is that fsaved option becomes unset.
The fix proposed is to add the value="on"
attribute in the template.
File: qmailadmin/html/mod_user.html
Locate line with input field "fsaved"
<input type="checkbox" name="fsaved"
value="on" ##i3>
Great find, but I don't think anyone is maintaining QmailAdmin at this point. I was heavily involved with development at one point, but used it for years without changes, and then moved to a managed hosting service with a cPanel interface.