Is it still under warranty?? We have a my15 Cherokee which we bought new from the dealer in 2017. It came with a 5 year warranty. I don't know if you'll have any satisfaction from FCA but you won't know until you ask. The fact that it's been serviced elsewhere other than Jeep may be fuel for them to say that it's been serviced incorrectly or they've used the wrong type of oil etc etc. As a mechanic, I service mine religiously every 6 months and I only use genuine parts and Penrite oil so if anything like this happens, I have receipts for everything proving it's been serviced correctly. So far we have had no problems with warranty issues.
If I was in your shoes and can't get any love from Jeep, I'd probably look for a 2nd hand motor although I don't think they'd be easy to find as they're still a relatively new model. Whichever path you choose it isn't going to be cheap either way.