I was having issues with importing a product csv that had made use of Advance Custom Fields. Upon import, I was forced to open and resave the product in order for it to display correctly. Using Claude, I was able to narrow it down. Here is the recap:

BUG: ACF button group and select fields inside Flexible Field layouts are not saving their mapped values when using "Select value for all records" — they delete the existing value and write an empty string instead of the selected option.

WORKAROUND: Using "Set with XPath" with a plain string value (e.g. square, h2) correctly writes the value.

ENVIRONMENT: WP All Import Pro + WP All Import ACF Addon, ACF Flexible Fields with button_group and select subfields, WordPress 6.7, PHP 8.

STEPS TO REPRODUCE: Map a button_group subfield inside a flexible field layout using "Select value for all records", run import, check database — field is empty.