Would you mind describing how you set up that filter?
edit: Using AdGuard for Safari, the following works for me under custom filtering rules:
||reddit.com^$document
||old.reddit.com^$document
@@||reddit.com/r/*/comments/*
@@||old.reddit.com/r/*/comments/*
@@||reddit.com/r/*/comments/*$document
@@||old.reddit.com/r/*/comments/*$document