I'm the OP on this topic. The problem never has been resolved, although the tech at
Amcrest Support hung in there with me for over a month before I finally threw in the towel. There's a simple workaround that takes care of my needs: I just set the cameras that have the problem to reboot daily. All of my other cameras reboot weekly.
The reboot takes place between 2AM and 4AM and is staggered so that the cameras (4) are not all offline at the same time. The cameras are back up and running after about a minute or less, so not much imagery is lost.
I can confirm what other posters have said: the latest firmware version (21-07-12) available on the Amcrest website does not fix the problem.
I'm currently running 13 of these cameras. Other than this glitch, they have had near zero problems over several years now. I'm very happy with them and may buy more. Daytime and nighttime imagery and audio all are excellent for a $55 camera, IMO.