HI,
I am not sure, if this belongs here, but from what it happens only when using your theme.
I am using the Backorder Manager Pro plugin
https://codecanyon.net/item/woocommerce-backorder-manager/19624014
It has a setting that disables sending of backorder/out-of-stock email notifications
https://ibb.co/nLm6zY5
However this does not work when using XStore theme, the backorder plugin shows fatal errors and is still sending out-of-stock messages – see example in PM
However this happens only in your theme and I have not received these errors whe using others.
I am assuming that since your theme does have a built-in Email Builder (which I do not use), it must affect sending emails somehow.
Can you please check?
Regards