It’s interestingly enough related to the square brackets in the shortcode that I’m trying to store.
The following works fine:
'shortcode' => $user_monthly_fee['shortcode'] ?:
sprintf('pricing_user_monthly_fee id="%s"', uniqid()),
);
While the following doesn’t store the shortcode:
'shortcode' => $user_monthly_fee['shortcode'] ?:
sprintf('[pricing_user_monthly_fee id="%s"]', uniqid()),
);