iPhone: some emails in “All Sent items” showing as unread

On my iPhone I have an IMAP account for yahoo.

when I send an email it is saved in my (yahoo server) [sent] items folder.

It also appears in my iPhone “All Sent Items” folder. However, some messages get marked as unread and others do not have an unread status (as I’d expect in a Sent folder, it’s not my inbox after all).

If if I open the message in my Sent items there is no blue dot.

If I open the same message in my All Sent items there is a blue dot which then vanishes after a short period of time. This then makes the unread status null and the blue dot, and the subsequent unread message count resets.

This behaviour would appear to be a bug. Is there a permanent fix; or a process to reset all unread statuses in the all sent items folder; or a configuration setting that needs to be set?