Setting up intermittent warning buzzer

I am using a couple of ESP8266 boards with Home Assistant for sensors and alarms.  All is working fine.

I have just added a 3.3v piezo buzzer to pin 8 of one of my boards to use as an alarm pending warning.  The buzzer works fine if set up as a continuous sounder.  I would like to set it up as an intermittent warning buzzer.  The following code works fine:

      switches:
        - pin: 8
          name: 'Warning buzzer'
          momentary: 500
          pause: 500
          repeat: 15

 However, I would like a continuous buzzer that stays on until the switch is switched off.  I have tried:

      switches:
        - pin: 8
          name: 'Warning buzzer'
          momentary: 500
          pause: 500
          repeat: -1

 As per the documentation.  With this setup, the buzzer sounds only once and then stops.  The switch in HA acts as a momentary switch, and turns off about a second after turning on.

Any idea what I am doing wrong?