I did some quick testing and for some reason 'min' => 0, forces the field to save as an array. I haven’t dug into the reason yet, but I thought it might help you to know.

Piklist settings are saved using the WordPress settings api, which saves all data as an array.

In the Piklist Demos we replicate the core WordPress SETTINGS > READING page, which saves data as separate values. Here are the examples from the Demos.

You would need to create a form, like this:
https://github.com/piklist/piklist/blob/develop/add-ons/piklist-demos/parts/forms/reading-settings.php

And then create an admin page where you would embed your form:
https://github.com/piklist/piklist/blob/c16ebde4eef529ad97337c83ac54c3192e0cddbd/add-ons/piklist-demos/parts/admin-pages/reading-settings.php#L26

I suggest you look at the piklist_save_fields hook, that should help you look at your field data before save.

Using the standard WordPress filter wp_insert_post_data, will also help you look at data before it’s saved:
https://codex.wordpress.org/Plugin_API/Filter_Reference/wp_insert_post_data

We’ll work on fixing the warnings. The error is in the Demo add-on. As long as you don’t activate it you should be fine.

I’m still confused. I don’t see a reference to a hidden field.

Yup… that is the usually the case: https://piklist.github.io/docs/faq/fields-not-saving/

Have you tried remove_shortcode:
https://developer.wordpress.org/reference/functions/remove_shortcode/

You can create a hidden field by setting the field type to hidden.

https://piklist.github.io/docs/fields/types/hidden/