So this is an issue that I have never seen or dealt with.
2017 hyundai elantra. Runs fine however, if it idles in park for to long when it is put into drive it will only have 4th gear. Put it back in park and shut off the car let it sit for a few minutes when I restart it. It will run fine again with no issues. I am an uber driver soo I tend to live my life in park while idling. Any ideas on what is wrong?

 

Whatever is causing the problem is getting erased and allowing the computer to reset when you restart the vehicle.
The absence of a voltage /signal is sometimes what causes the vehicle to go into "Limp mode" so it doesn't do any damage and can still be driven to a service facility.
Most of the time it will store a code in the computer. That should be checked to see if there is any code to provide some direction as to where to look and check..

 

Do you have a Check Engine Light illuminated? This problem is not new and is related to the transmission Input/Output speed sensor going intermittent before it fails completely and leaves the car in the Limp mode Hanky mentions. There are many reports to be found on the internet about it. The part runs about $30.00. If you're handy and can do the work yourself go for it. If not, check around and get a quote to replace it.