i had also same problem 5-6 times. i am try to add a text in my userprofile wall. and it is adding deleted user.
I have not looked why, but I have verified that with PHP 5.6 it never happens and with PHP 7.0 or higher it sometimes happens.
This problem is occasional and difficult to reproduce.
I don't know if the most modern version of newsfeed (updated on June 17, 2020) that is at https://github.com/oxwall/newsfeed fixes it, but installing it won't hurt. :)