Bulk Price Editor changes Shopify product prices directly. It does not create a Shopify checkout discount and cannot block discount codes. An eligible code can therefore discount an already reduced sale price.
For example, a 20% sale changes a price of 100 to 80. A further 10% product discount would reduce 80 to 72, subject to the code's eligibility and Shopify's calculation. Changing Shopify discount-combination settings does not turn the app's price change into a checkout discount that those settings can prevent from combining.
Limit an amount-off-products code
Manage discount eligibility in Shopify. For an Amount off products code, use Applies to > Specific collections and choose a collection containing only products that should receive that code. See Shopify's amount-off discount guide.
Review the collection's actual products and test both a sale item and a regular-price item at checkout. If a product has both discounted and regular-price variants, collection membership alone does not establish that only the intended variants can use the code.
This collection approach is for product discounts that Shopify allows you to scope to collections. It is not a universal exclusion for order-wide discounts, shipping codes or discounts controlled by other apps. Review those separately in Shopify. If you want a code unavailable to everyone during the promotion, consider deactivating that code for the period instead.
Why empty compare-at prices can include sale products
A collection using Compare at price > is not set is not a reliable general definition of products without a sale. In a product-based collection, one variant with an empty compare-at value can make the whole product match, even when another variant is discounted. A nonempty compare-at value is also not proof of a current discount. See Shopify's collection conditions.
Use a collection containing only products allowed to receive the code, and check which products it contains. If you need to remove stale crossed-out prices, follow remove crossed-out prices without changing selling prices for the intended products. Do not reset compare-at values on an active sale just to make a collection rule match.
If a code still discounts an unintended item, share the code type, eligibility settings and one example product with support.