Products sort by Price not woking correctly with simple and configurable products

Recently, a client noticed a problem with the products sort by Price function in the category page.

For example, the user selects Sort by Price Ascending. The system will return the configurable products (not in price order) and then the simple products (in the correct ordering). It’s like the configurable products have no price.

Checking the database, I found configurable products in the catalog_product_index_price table have 0.000000 in the price fields, while their simple products have the correct prices in that fields.

All the simple products have a sale price and a special price along with a defined date range.

I checked the products and the configurable product’s price field is disabled.

I have run reindex and cleared/flushed the cache, tried to change the Product Price Indexer to Update on Save and save a product but nothing changes.

All the posts I read point to some indexing issue but I can not locate it.

Any help or guidace on solving this?

Thank you in advance.