So Google is on a mission to increase battery life on Android phones, but they're only picking the low hanging fruit. When the screen is off, apps stop unless they are executing an occasional background task. CycleBike needs to keep running to save a breadcrumb track of position, even when the screen is off. In the past, apps could request a “wake-lock” to keep executing. Now, with Android Oreo and Pie, Google forces apps to turn off the GPS receiver and Bluetooth radio. The workaround is to put these functions in a Foreground Service. The developer also has to ask the user to disable ”battery optimization”, and requires a notification that these Services are active. A Bluetooth radio can be looking for only a few specific devices.
In fact, my observation is that the screen back-light uses two thirds of the battery life. The GPS receiver and Bluetooth radio only account for a fraction of this. Bigger, brighter screens use more battery.
This demands a lot of work from developers for a small return in battery life. Not that anybody cares.