If you create a field with a capitalised name, e.g. Test_field then delete that field and create a new one with the same name but in all lower case e.g. test_field you cannot enter any data into the new field, after clicking update the data you enter will disappear and not be saved.
Checking the field name entry contains only lowercase letters when creating a field seems like a quick fix for this.
Also deleting any field and then creating a new one with the same name fills the field with the data from the deleted copy. Not really so much of an issue but a bit quirky.
The first issue occurs because ACF saves the field name in case-insensitive format but loads it in case-sensitive format. Could you please open a new ticket so this issue can be passed directly to the plugin author? You can open a new ticket here: https://support.advancedcustomfields.com/new-ticket.
Regarding your second issue, it’s because ACF won’t delete the post meta (custom field) when you’re deleting the field. This will prevent deleting post meta that is still in use.
I hope this makes sense 🙂
Ok, yep that makes sense. I’ve opened that ticket.