[BUG] Flatten on projected fragment yields incorrected result in GAE

[sighingnow]

Describe the bug

import graphscope
from graphscope.dataset import load_ldbc

graph = load_ldbc()

pg = graph.project(vertices={"post": [], "tag":[]}, edges={"hasTag": []})

pr_context = graphscope.pagerank_nx(pg, alpha=0.85, max_iter=100, tol=1e-06)
print(pr_context.to_dataframe(
    selector={"id": "v.id", "dist": "r"}
).sort_values(by="id"))

pg = graph.project(vertices={"post": [], "tag":[]}, edges={"hasTag": []})
c = graphscope.hits(pg)
print(c.schema)
c.to_dataframe(
    selector={"id": "v.id", "auth": "r.auth", "hub": "r.hub"}
).sort_values(by="id")

The result contains vertex not in "post" and "tag", and the result set size is wrong.

Additional context

Reported by @hanjf12

[sighingnow]

See also: v6d-io/v6d#1223