After 2 years this defect was initially reported, it is still open with Arducopter 3.6.9 along with the IR-Lock sensor for precision landing.
When PLND_EST_TYPE is set to 1, the copter gets out of control and goes into a uncontrollable spiral when Precision Loiter is enabled once the IR beacon gets in sight. Unlike during Land mode where you can abort the spiraling by switching into another flight mode, disabling precision loiter does not bring back control.
As long as this defect has not been fixed, please do not enable the kalman filter and keep PLND_EST_TYPE set to 0.
Link to the issue and latest log: