SQL error on rearranging taxonomy terms via GUI

We just added a new term to an existing taxonomy vocabulary, which worked fine. But when I go to the "list terms" page and try to move the new term to a different place within the list (keeping the same parent term), I get this error when I try to save it:

Error
SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '33-31' for key 'PRIMARY': UPDATE {taxonomy_term_hierarchy} SET `parent`=:db_update_placeholder_0 WHERE (`tid` = :db_condition_placeholder_0) ; Array ( [:db_update_placeholder_0] => 31 [:db_condition_placeholder_0] => 33 )

Also, when I reload the "List terms" page, the parent term was displayed in a different place.

Oh, and the terms that it's complaining about weren't touched in the editing.

I am running the latest version of Backdrop (1.31.0) and all modules.

Any idea what's going on?