1,5 Km is a bit much for a BT Connection
"Bluetooth 1.0 can link up to eight devices within a radius of about 33 feet with a capacity, or bandwidth, of 700Kbps. A later revision of the Bluetooth specification, Bluetooth 2.0, can transmit at up 2.1Mbps within a radius of about 100 feet. Bluetooth 3.0, adopted in 2009, increased bandwidth to 24Mbps, while the Bluetooth 4.0 specification, adopted in 2010, has a theoretical range of up to 200 feet. The Bluetooth specification stipulates a minimum range of 33 feet, but the maximum range is determined only by the output power of the device."
Maybe the F5 has a built in buffer for audio playback, so it won't cut out if the BT connection has issues, and it was simply playing the audio stream that had already been buffered.
Were you able to change tunes whilst on the move ?