If you force close your Tile app from the recent apps page, you will see the following notification:

When you force close the Tile app, it is no longer running in the background.
This means you will not be able to use features like Find Your Phone and Smart Alerts, and your Tiles will not be getting location updates.
Relaunch the app to use all of Tile’s available features.

Learn how to check if your Tile app is running in the background.