Deleted files are still available in "Recently Deleted” within the iOS Files app. This offers a way to restore the files if you accidentally deleted them, and is automatically cleared after 30 days.
If you find Doppler is still taking up storage space when no music is visible, this is likely the cause.
Manually clearing "Recently Deleted" files
- Open the iOS "Files" app on your iPhone
- Within the "Browse tab", select "Recently Deleted"
- Tap the options button (•••) in the top right of the screen and select "Select"
- Select the files you want to permanently delete and tap "Delete" (alternatively tap "Delete All" at the bottom of the screen)