Aloha Ying,
Ahhh — I think I got it π
I set the buttons up using the Dynamic Data options in the WordPress sidebar, as opposed to using the Dynamic options that appear directly above a selected element.

^^ aria labels with this dynamic data setup were not working.
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I reset all the dynamic options to the ‘old’ way and the aria labels now work perfectly π

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Am I not using the sidebar dynamic data options correctly?
You the best π
Thanks
_bill