Sending multiple push notifications as field "to" of type array fails

SDK Version: 40
Platforms(Android/iOS/web/all): Android and iOS

Sending multiple push notifications at once fails. I created the following post request as described on the docs (I send the request from a Wordpress website):

{
"title":"Title for push",
"subtitle":"Subtitle for iOS",
"body":"The body text, of course.",
"priority":"default",
"sound":"default",
"badge":xx,
"chandelId":"channel_name",
"to":["ExponentPushToken[xxx]","ExponentPushToken[xxx]","ExponentPushToken[xxx]","ExponentPushToken[xxx]","ExponentPushToken[xxx]", ... + 35 in total]
}

The problem is that when I make a request to expo servers with more than 16 push tokens in the field “to” I get an error: “VALIDATION_ERROR Field ‘to’ must be of type [array, string]”. But it is an array! When I simply send less than 15, it works… same array, less items.

Why more than 15 push tokens within the “to” field fails to send the push? What am I missing here?
Thanks