After routine service, & recall work done on my 2019 Subaru Outback, 11,000 miles, it has a very very fast idle which does not recycle down at any time. The recall was for a fuel pump .
Have you called the dealer and asked about this ? If it did it when you picked up the vehicle you should have said something then . Since you are in the warranty period the dealer should be the one to look at this .
In fact, having this new problem addressed by anyone other than the dealership could result in the OP’s factory warranty being cancelled.
During the warranty period, nobody but the dealership should be doing any electronic or mechanical repair work on a car, and to ask for recommended repairs in a public forum suggests that the OP might use a mechanic other than the dealership to address the problem.
If the dealership is unable to resolve the problem, then the OP needs to contact Subaru at the corporate level. Contact info can be found in the Owner’s Manual.
Most likely the mechanic left a vacuum line disconnected in the vapor recovery system, but only the dealers mechanic should take care of this. I’m sure if you call them, they will know exactly what happened and will fix it in a few seconds.
Leaving sensor disconnected could cause. Is check engine light on? Less likely, fuel pressure problem.