I see in the options where I can sort by a meta field in the Post query orderby settings, but not for the meta fields created by Advanced Custom Fields (https://wordpress.org/plugins/advanced-custom-fields/)...
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If your ACF custom field value save plain number or string you can use directly meta key under post grid settings, please use the option
Post query orderby > “Meta value(Number)” if your custom field value type is number otherwise use “Meta value”
Please watch my testing here with ACF plugin and post grid.
If this is still not working you can use filter hook to override query arguments,