How to reuse Specification<T> inside another Specification<F> with subquery?

I’m struggling with reusing a Specification<T> inside another one.

Entities:

• Group has a list of StudentGroupInfo.

• I already have a Specification<Group> that can filter Group by various conditions (joins, group by, having, etc.).

• Now I need to count all StudentGroupInfo for the groups that are matched by this Specification<Group>.

Repository:

public interface StudentGroupInfoRepository extends JpaRepository<StudentGroupInfo, Long> {
    long count(Specification<StudentGroupInfo> specification);
}

Attempt to reuse the group specification via subquery:

private Specification<StudentGroupInfo> buildSpecificationByGroupSpecification(
        Specification<Group> groupSpecification) {
    return (root, query, builder) -> {
        var subquery = query.subquery(Long.class);
        var groupRoot = subquery.from(Group.class);

        // how to apply the groupSpecification here?
        var groupPredicate = groupSpecification.toPredicate(groupRoot, query /* ??? */, builder);

        subquery.select(groupRoot.get(BaseEntity.Fields.id))
                .where(groupPredicate);
        return root.get(StudentGroupInfo.Fields.group).in(subquery);
    };
}

Problem:

If the groupSpecification is simple, it works. But if it contains joins, group by or having, Hibernate throws:

Caused by: org.hibernate.sql.ast.SqlTreeCreationException: Could not locate TableGroup - com.sdx.rootservice.model.group.Group(...)

I assume it happens because I pass the outer query into groupSpecification.toPredicate(...), while that Specification expects a root from the main query, not from a subquery.

Question:

How can I properly reuse an existing Specification<Group> inside a subquery in another specification, or do I need a different approach for it (e.g. refactor to join instead of subquery)?