I have checked the issue in my local setup and noticed that it is not actually a bug — WooCommerce is working as intended. I have clearly explained how the backorder status works for variation products below and also included the settings screenshots for your reference. Please review them, and if you have any further doubts, kindly let us know.
WooCommerce Backorder Settings (for Variations with Manage Stock)
When Manage stock is enabled for a variation and stock reaches 0, WooCommerce uses the Allow backorders option to decide what happens.
1. Allow
Variation is purchasable, even with stock 0.
Customer can add it to cart and checkout.
On product page / listing:
Status still shows “In stock” (not “Backorder”).
Customer is not notified that it’s actually a backorder.
Only you (admin) know stock is 0 but purchases are allowed.
2. Allow, but notify
Variation is also purchasable, even with stock 0.
Customer can add to cart and checkout.
On product page:
WooCommerce will display “Available on backorder”.
Customers know the item is not physically in stock.
On listing page (shop/archive grid):
Parent product may show “Available on backorder”
https://drive.google.com/file/d/1Xgdwc-fB_pDQOZv64XRdiU5E0udPgcIc/view?usp=drivesdk
https://drive.google.com/file/d/1au-Ty_ZTwBIUPJFVA5v0G0Tivag0cXFB/view?usp=drivesdk
