Cookies stored using JavaScript being retrieved incorrectly with PHP

I have a web app built on Wordpress that stores a cookie using JavaScript:

document.cookie = "storedcookie="+cookie+"; path=/";
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Later, it retrieves the cookie using PHP:

function prefill_fields ( $fields ) {
    $fields['default'] = $_COOKIE["storedcookie"];
    return $fields;
}
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99% of the time this process works without any issues. However, for certain users, something strange happens:

If the cookie originally stored is: "~thisisacookie~thisisthesecondpart~thisisthefinalpart~"

Then, what is later retrieved will be: "~thisiy"

Basically, the first 5 or 6 characters, followed by a 'y'.

The users who usually experience this problem are often using iOS using mobile Safari. I have not been able to replicate this problem on any iOS device (iPhone 8 and onwards).

Does anyone have any clue what could be causing this issue? Many thanks.

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