I can only find reference to this filter in your plugin and WooCommerce itself:
Searching 3076 files for "woocommerce_can_reduce_order_stock"
<removed for anonymity>/wp-content/plugins/reduce-stock-of-manual-orders-for-woocommerce-master/reduce-stock-of-manual-orders-for-woocommerce.php:
47
48 // Make sure that is possible to reduce order stock.
49: add_filter( 'woocommerce_can_reduce_order_stock', '__return_true', 999 );
50
51 // Reduce or increase order stock when changing the order status on the admin screen.
<removed for anonymity>/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-order.php:
2625 /**
2626 * Reduce stock levels for all line items in the order.
2627: * Runs if stock management is enabled, but can be disabled on per-order basis by extensions @since 2.4.0 via woocommerce_can_reduce_order_stock hook.
2628 */
2629 public function reduce_order_stock() {
2630: if ( 'yes' === get_option( 'woocommerce_manage_stock' ) && apply_filters( 'woocommerce_can_reduce_order_stock', true, $this ) && sizeof( $this->get_items() ) > 0 ) {
2631 foreach ( $this->get_items() as $item ) {
2632 if ( $item['product_id'] > 0 ) {
3 matches across 2 files
Nothing in the functions.php file either.
“Manage Stock” option is enabled in WooCommerce.
We have “Manage stock?” enabled at variations level.
Any other ideas much appreciated.