Not sure when this crept in but current Version 18.104.22.168 the default value is not showing for a select box in the admin.
I just tried a select field and it seems to be working for me.
Do you have another field on the same page with the same name? There could also be a filter that’s causing a problem with the field’s default value, try disabling plugins and switching to one of the default WP themes to narrow down the cause.
my staging/testing version has no issue and is using Version 5.3.0 ! I’ll roll back to this version on my dev site and see what happens.
If that will work, but I’m guessing that sooner or later you’re going to want to update the plugin and there does not appear to be a problem with it when running nothing but ACF on a default theme. If we can narrow down the cause and it’s a bug in ACF that happens when combined with something else then we can alert the developer.