Hey Calebrachoa,
Thanks for those details, however if you're not absolutely certain this app is using mobile data unexpectedly, please use the steps provided in our last response in order to determine with certainty if this is the case. We don't want to make assumptions while troubleshooting.
Also, you'll know if you're using CarPlay because the CarPlay system will appear on your car's display, instead of the built in OS from the manufacturer, it also does look like the 2017 - 2019 Fiesta supports CarPlay as outlined here: iOS - CarPlay - Available Models - Apple
Next, you mentioned mobile data being off, so to clarify, are you saying that you go to Settings > Cellular and turn off cellular data for this app prior to getting in the car?
Keep us posted on the results after using the steps in our previous response to determine with certainty that mobile data is being used by this app while it's not in use. Also, if the issue is isolated only to this particular app, make sure it's updated. If it is, try reinstalling the app and testing further.
Take care!