Sir, i am using magisk manager 8.0.2 and beta 21 magisk module
and yes bettery optimization disabled too...
every time when i open the app he give me this message
Magisk Module Installed!
Please reboot to finish the installation
when i reboot the device again that message showing on opening the app...
what happend to me please see in this video link
Try to use stable magisk 20.4 instead of 21, might be the culprit