Interesting update:
Seems like a different maximum/capped score for the different model options on AutoML:
0.6 max on Low Latency option
0.5 max on Best Tradeoff option
Both models work well in TensorFlow.JS format, with no cap on scores - so its definitely specific to the TFLite model conversion in AutoML Vision.
Additionally, I ran the export again today and still have the same issue - so it has not been resolved yet.