Stock Status goes to ‘Out of Stock’ even when backorders enabled

I have “Backorders: Allow Qty Below 0 and Notify Customer” but when a product is sold and quantity drops to 0, -1, -2, or less, Stock Status will change to ‘Out of Stock’.

When ‘Out of Stock’, then the product can’t be purchased, so I have to manually change it back to ‘In Stock’.

It seems inconsistent. It usually changes to ‘Out of Stock’, but not always.

I’ve tried to set Out-of-Stock Threshold to -99 but it seems to only allow 0 and higher.

I have Amasty Extension Custom Stock Status, but it doesn’t seem to have any effect on this issue.