How to customise / set localization text when stream is not loaded

[deplorableword]

I am trying to customise the text that is displayed when a stream is not loaded.

The live stream or video file are not yet ready.

I've attemped to do this by passing in some text but I can't get it working the text remains the same, any idea what I'm doing wrong

<mux-player
        id="muxPlayer"
        stream-type="live"
        playback-id="SEmedYlPSdRJDoSdhY028bCWU86DNw9xXh00MI2601mfq00"
        accent-color="#969696"
        primary-color="#FAFF3D"
        autoplay="muted"
        player-language="en"
        localized-ui='{
            "en": {
                "playback": {
                    "notReady": "BACK SOON!"
                }
            }
        }'>
    </mux-player>

Any idea what I'm doing wrong?