Goodday rudolfl,
The only thing that i can think of is that your save_post_product() function is called before the acf/save_post function. if that is the case it posiable that your value is overwritten.
I recommend setting your save_post_product() to mabye prioity 999 to check if this is true?
I hope this will help if not can you send me the full function of you doing this?
With kind regards,
Mauro
