Turtle serializer outputs undeclared prefix with same domain base

[rodrigosetti]

Reproducing code (v8.0.0a0):

from rdflib import Graph, URIRef, Literal

g = Graph()
g.add(
    (
        URIRef("https://example.com/subject"),
        URIRef("https://example.com/p/predicate"),
        Literal("object"),
    )
)
print(g.serialize(format="turtle", base="https://example.com/"))

@base <[https://example.com/>](https://example.com/%3E) .
@prefix ns1: <[https://example.com/p/>](https://example.com/p/%3E) .

<subject> ns2:predicate "object" .

Please note "ns1" declared, but "ns2" used in the triple.