I'm creating hosted page to update subscription.
Using additional param to identify subscription (just do not want to store hosted page IDs in data base).
And it was work fine. But now additional param  don't attaches to redirect url.

All other things works like before.

{
"plan": {
      "plan_code":"Platinum"
   },
"reference_id":"61XLbadKrvHdANOQpzfx5zagAZ9FPoUYct",
"additional_param":"reference_id=61XLbadKrvHdANOQpzfx5zagAZ9FPoUYct",
"redirect_url":"mysite.local",
"subscription_id":"246149000000125439"
}