Okay I have a pretty crispy workaround, but it works.
I am using ACF Pro 5.7.11
In the old thread find the first reply by shaun where he has a code block. Here’s the link:
https://support.advancedcustomfields.com/forums/topic/custom-fields-on-woocommerce-product-variations/
Use that code in functions.php or wherever.
In ACF setup whatever custom fields you need, but in the rule select product instead of product_variation because that option isn’t present.
Go to the ACF Tools page and export the .json of your custom fields. Open that file with a text editor and scroll down to the “locations” section. Simply change the “value” from “product” to “product_variation” and then on that same tools page re-upload the file. You should see custom fields in your product variations drop downs now on variable products.
