Ever since about ACF 5.3.1, the select field no longer saves the default value properly. I have noticed this on the last 5 or so WP sites I have developed using WP 4.4.0 – 4.4.2. After saving the field, the default value will revert to either ‘0’ or ‘0 : <value>’. Previously it would just save the value.
Do you mean when saving the field group or when saving the value on a post or page or wherever the group is location. I’ve tried these with the latest version 5.3.4 and it seems to be working.
I mean when saving the field group. I’ll open a ticket, thank you.