Fix subdomain inheritance for nested blueprints.

Fixes #4834

tests/test_blueprints.py

@@ -950,6 +950,27 @@ def test_nesting_url_prefixes(
     assert response.status_code == 200
 
 
+def test_nesting_subdomains(app, client) -> None:
+    subdomain = "api"
+    parent = flask.Blueprint("parent", __name__)
+    child = flask.Blueprint("child", __name__)
+
+    @child.route("/child/")
+    def index():
+        return "child"
+
+    parent.register_blueprint(child)
+    app.register_blueprint(parent, subdomain=subdomain)
+
+    client.allow_subdomain_redirects = True
+
+    domain_name = "domain.tld"
+    app.config["SERVER_NAME"] = domain_name
+    response = client.get("/child/", base_url="http://api." + domain_name)
+
+    assert response.status_code == 200
+
+
 def test_unique_blueprint_names(app, client) -> None:
     bp = flask.Blueprint("bp", __name__)
     bp2 = flask.Blueprint("bp", __name__)