As Google continues to make it harder for applications to share data, you may have to move your offline maps and GPX save location to a MyTrails-specific folder rather than the traditional
Several users have reported that using a folder like
android/media/com.frogsparks.mytrails (inside the main device storage, presumably also on a removable storage) works.
If you’re not using offline maps (only GPX saved tracks), you can probably stick with your usual save location, but you need to switch to the standard Android file picker (disable
Use internal file picker in MyTrails > Preferences > Other).
For more general info, see this older post.