Missing author names

Hi,
All members are exiting users. When a new post is created via email, some user’s display name is missing in the timestamp. I notice too that when this happens, the category is being assigned to the previous default category rather than the current default category.

At first I thought is was something in the display name or perhaps a user role, but I havne’t been able to see a pattern. It’s seems like a random issue.

Any help is greatly appreciated.

https://wordpress.org/plugins/postie/