Some devices released more than 3-4 years ago are starting to experience some performance issues, ranging from slow-downs to the game unexpectedly closing.
This can be for a variety of reasons, including coding errors (memory leaks) in the game, the demands of updated systems/services that our game requires (e.g. Unity), or just the general strain caused by the addition of new features that we update the game with.
For now it's likely that you may be able to continue playing short sessions of the game, and restarting every so often, but long term this may become more of an issue.
Note: For our part, we will continue to try and improve the memory usage with every update however this problem may not be entirely resolved.