Preventing BLE reconnection after pairing. Single-use connection.

can you please tell me that exact steps you took to get the change to work

Sure. I connected with the Web IDE in Chrome, entered NRF.setAddress("f6:f6:f6:f6:f6:f6 random"); on the left-hand side of the IDE, disconnected, and then checked with NRF Connect on an Android phone and sure enough it had changed.

NRF.setAddress will apply only until the device restarts (even after save) - so if you're resetting it by disconnecting the battery then yes, the old address will remain.

If you add the NRF.setAddress command to the onInit function you should be fine though.