On-Hold emails are only sent when an order is manually set to On Hold from a different status in WooCommerce.
Snippet Type
Execute on Child Sites
Snippet
https://github.com/mainwp/Code-Snippets-Functions/blob/master/Execute%20a%20function%20on%20a%20child%20site/WooCommerce/on-hold-order-email-sent-only-manual-change.txt