It is possible to get a bookmark into a state where it can't be deleted. My setup includes the KJVS and Gill's commentary. Example steps to reproduce:

1. Go to read Romans 9:13
2. Bookmark that verse
3. Tap on the "13" and choose "Analyze Verse"
4. Tap on "Commentary" and look at Gill's commentary.
5. Within the commentary, tap on the reference to Malachi 1:2.
6. When that reference expands, tap on the "KJVS".
7. Select "Bookmark this Verse" from the popup window.
8. Tap the "back" button to exit out of the verse analysis.
9. Tap on "Bookmarks".
10. Delete both of the bookmarks you just added.
11. Close the app.
12. Reopen the app and make sure that "Bookmarks" is selected. Notice that Romans 9:13 was successfully deleted, which Malachi 1:2 is still there.

Workaround:
The only workaround I know of is to backup all of your bookmarks, send an email to support to have them fix the offending bookmarks, and then restore your bookmarks.

Excellent catch -- outstanding work tracking this down.

Thank you.