Uh-oh. According to a support doc released yesterday, T-Mobile has paused the Optimus L9's update to Android 4.1.2 Jelly Bean because of the new software's less-than-stellar stability. Several users have reported significant battery drain, difficulty receiving calls, and frequent app crashes. Accordingly, T-Mobile wants to "improve performance" before resuming the OTA upgrade.
T-Mobile announced and began rolling out the Optimus L9 Jelly Bean upgrade on April 22nd, so it's somewhat surprising it took so long to identify problems with the update. Still, better late than never.
As far as ETAs go, T-Mobile unfortunately doesn't know when the fixed update will begin rolling out. Stay tuned for the latest.