Thanks for the question.
Do you have battery optimization enabled (I think it’s enabled by default). Oneplus, like many other manufacturers use a battery optimization technique that stops apps that run in the background. Even though the app correctly specifies itself as background app that should not be stopped.
Related questions can be found in the oneplus forum as well, for example here and here.
The proposed solution is to go to settings and disable aggressive battery optimization and also add an exception for Kurviger. Then you need to change “Manage recent apps” to normal mode. If you still see issues, you can also try to go to recent apps and click the lock next to the Kurviger app.
Currently battery optimization on newer Android versions is a big issue with some manufacturer modifications. Other OEMs like Xiaomi, Huawei, and others struggle with this issue as well.
Please let us know if you were able to fix it and which settings were required to change, so we can advise other Oneplus users in the future about the correct workflow.