Yes, I have similar issue with reserved IPv6. But I have noticed that issue appeared in php 8.4.
For IP:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

The code like this:

filter_var(
            $ip,
            FILTER_VALIDATE_IP,
            FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE
        )

for PHP 8.3 returns bool(false)
but for PHP 8.4 returns

2001:0db8:85a3:0000:0000:8a2e:0370:7334

So does not filter it.

I guess it might be problem of some php settings, but I haven't found any.