Some device manufacturers use a non-standard method to save battery by essentially killing any App that should be set to work in the background so this removes the permissions that we need.
Oppo & Vivo (most devices under the Vivo brand are manufactured by Oppo) are notorious for their aggressive battery optimization so we have to exempt Boomerang Parental Control (for kids) or Manage SPIN (for adults) from this aggressive battery optimization. Thankfully, there is a method that works on most devices to have the permissions "stick."
For a list of fully supported Oppo devices, please visit this post:
Ensure that Accessibility is Toggled On
- Settings > More Settings > Accessibility > Set Boomerang Accessibility Permission On
Set Boomerang for the High background power consumption
- Settings > Battery > High background power consumption > Toggle On Boomerang