This happened to me while editing, the following seems to be the minimum needed reproduce the problem:

• Find an existing statement with a reference
• Click "add" to start adding a second statement for the property
• Click "edit" to start editing the existing statement
• Click "remove" in the reference section to remove the existing reference
• Click "add reference" to start adding a new reference to the existing statement
• Click "cancel" to cancel the addition of the second statement

Expected behaviour: The second (new) statement is removed and the existing statement remains as it is.
Actual behaviour: Both statements are removed from the page.

It doesn't actually delete the existing statement, it comes back after refreshing the page, but it was nevertheless annoying because my changes to the existing statement vanished before I could save them.