I don’t think there is enough information here for me to help you, so this is a guess.
You say that text fields work fine and the checkbox does not. I am assuming that you are somehow using the values stored in an ACF field for these but you’re not using get_field() to get the values. Text fields are stored as simple text fields the same way that WP stores custom fields. Checkbox fields are stored as serialized arrays in the database. If you’re using anything other than get_field() to get these values then you may be dealing with the returned value in the wrong way.
More information would probably be helpful, like how you’re storing the checkbox value and how you are trying to get those values and display them in your custom form.
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