Sorry I wasn’t clear. Now, the back-off is not working in lmic and it will continue to send join request as fast as it is allowed by region settings. Lmic do not respect specification on this point, this is a bug.
@descartes is right you can try different hack to reduce join number when you receive the EV_JOIN_FAILED (maybe reset lmic and sleep for 24h)
(Sorry if i am not clear, English is hard for me)