Restarting using the "System Restart" should not harm the RokuTV or any Roku device as it is simply a device reboot. It additionally clears the system/memory cache upon restart so actually a recommended course of action.
I use System Restart regularly on all my devices. (especially those used for testing when responding to user issues.) You don't need to worry about using System Restart "too much". It is just a reboot.
Maybe try the same reboot on the device you are using to initiate the mirror (the source). If you are using your phone, try rebooting the phone and not the Roku. If you find that rebooting the phone also fixes issue, then the culprit is simply a loss of communication between the source (phone) and client (roku).
I am sure future builds to the OS will address issues as they become known and affect a greater number of devices, but honestly, it is more of learning how to use "workarounds" to accomplish the task you are trying to achieve. I also think it may probably be more difficult to address the screen mirroring because they have to account for all the sources that are originating the mirroring. I know from web casting that some of my Android apps no longer work properly because some of the streaming sites changed the source stream format and Roku doesn't recognize them. I just had to find new source sites that were still compatible with the Roku.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941