Preventive transmission service?

Yes, it is a good idea to service the transmission. Here is a guide to help and the instructions on how to service your car in the images below.

Specified fluid: SUBARU CVT OIL FOR LINEARTRONIC

Caution:
Always use specified CVTF. Using other fluid will cause malfunction.

Although CVTF filter is a maintenance-free part, replace it if a large quantity of wear debris and metal particles are found in CVTF and CVTF filter.

This guide can help walk you through the steps:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Check out the images (below). Please let us know if you need anything else to get the problem fixed.