The Windows path I referenced is the Shotcut “system” area. Export presets can also be stored in a user’s profile in their home directory. What happens if you put the DNxHR preset in there instead of the system area? This post might help locate where the user preset area is on your distro. If you find the “presets” folder under your home directory, there should be another directory under it called “encode” where custom export settings can go.