In other words:

backend displays both params and attribs fields.
frontend displays params fields only.
both frontend and backend save data from attribs fields only.

Why did you close this?

Sorry, re-opened.

Yes, /components/com_content/view/form/tmpl/edit.php uses the code:

<?php foreach ($this->form->getFieldsets('params') as $name => $fieldSet) : ?>

So it displays params fields only, not attribs. But these params fields will be not saved since table uses attribs.

I can confirm this. Execpt for displaying custom fields, there is also a problem with saving.

Now it is impossible to create a plugin that save custom fields in the parameters for front- and back-end. The only way to do it now is creating a complete new fieldset and let the plugin save data in its own database table.
If you just want to add a simple custom text field this is overkill.

For the back-end you can do something like this:

<fieldset name="params">
    <field
        name="test"
        type="text"
        label="Text field"
        description="some text field"
    />
</fieldset>

The content of this field will be save inside the params row of the com_content table. Not sure way, but for the front-end you have to use a fieldset called attribs.

Note
I try to explain this as good as possible. If it is still not clear I can create a simple plugin to show the problem I have.

As the description of the option states that it is only to display them on the admin I am going to close this as expected behaviour and as it is now two years old clearly everyone else thinks it is to be expected.