Need to upgrade to latest stable
While raising a bug to K9-Mail for this application, they pointed out the fact that /e/ appears to be using a Beta version.
Also, is Mail a straight fork from K9-Mail or did you make any changes? I am asking because from that previous issue, they were not very keen on investigating fork issues.
But anyway, the stable version 5.600 is a year old, and should be pushed to /e/.