What I've noticed on Android is that different manufacturers have different ways to deal with prolonging battery life. And the recent Android versions have even more ways that the Android system limits apps and background tasks.
Some phones will put background apps to sleep, some will disable certain functions (like GPS), some will limit or disable background data, etc. All of these can have an affect on Alexa's ability to report your location to Amazon's servers, which in turn will run your routines. In other words, geofencing might not work depending on how long it's been since you opened the Alexa app, and whether the app is in the background, closed, or put to sleep, and what Android decides to do with it.