I use module v1.0.9 for my Magento 2.2.6. I set Pre Order option "When Product Become Out Of Stock" as this style is still in stock. Then I change one size to out of stock and the Pre Order button doesn't appear.
The PreOrder button just shows all the child products are out of stock.
How to solve this error?
The issue happens due to the fact that the module doesn't check child product, the function isAvailable() doesn't work on Magento 2.2.x.
To resolve this issue, please edit the file as below or download the attached file and overwrite the current file on your site.