Strange problem with setField() method and ExtraMeta field in Page Extension

Questions
1

Silverstripe 4.1:

Hi!
I don’t use ExtraMeta field in Page to store Extra Meta Tags but I use it to store some information in json format, so I disabled it with

public function updateCMSFields(FieldList $fields) {
        $meta_description = $fields->dataFieldByName("MetaDescription");
        $fields->removeFieldsFromTab("Root.Main", array("Metadata"));
        [...]
}

then I “override” ExtraMeta field in private static $db:

private static $db = array(
        "ExtraMeta" => JSONText::class,
);

I have no problem at the database level: the information is correctly recorded in the database every time I make changes from the CMS. But when I want to get the property ExtraMeta with

$this->owner->ExtraMeta

in Page Extension, I get the old value (it will be update only when I update other property of the page, like Description, Content, Title, etc).

It seems that the object $this is cached, so much so that even the property “LastEdited” remains unchanged.

How can I solve this problem? How can I invalidate the fields cache when I use the setField method to write on the database?

Thanks for the attention!