As the request with LS_op=destroy is part of the internal communication protocol, in principle, you may expect that the JavaScript Client Library will issue that request or not, depending on minor changes in your application code, as in your case.

As said, the fact that a request with LS_op=destroy causes a "sync error" is also to be expected in many cases and it is not an issue.
So, this "sync error" should be considered as a low-level error which is not an error at application-level.

I will check with my colleagues if the behavior you have observed is perfectly consistent with the library implementation.