Facebook graph page/post insights – permissions error

I have 2 facebook pages where im admin on, lets call them page_a and page_b

if i run this for page_a it works as expected:

HTTP GET -> {PAGE_A_ID}/insights?metric=page_fans_country


{
  "data": [
    {
      "name": "page_fans_country",
      "period": "day",
      "values": [
        {
          "value": {
            "AR": 4,
            "AT": 13,
            "AU": 7,
            "BE": 16,
            ....

the page access token i use have the following scopes: read_insights, manage_pages, pages_show_list and public_profile

but when i then try to query details about a post i get this

HTTP GET -> {PAGE_A_ID}_{PAGE_A_POSTID}/insights?metric=post_impressions,post_impressions_unique,post_video_views,post_video_views_unique&period=lifetime

{
  "error": {
    "message": "Permissions error",
    "type": "OAuthException",
    "code": 200,
    "error_subcode": 1504029,
    "is_transient": false,
    "error_user_title": "User doesn't have enough permissions to load insights",
    "error_user_msg": "You do not have enough permission to view the metric.",
    "fbtrace_id": "..."
  }
}

it works when i do the same for page_b

HTTP GET -> {PAGE_B_ID}_{PAGE_B_POSTID}/insights?metric=post_impressions,post_impressions_unique,post_video_views,post_video_views_unique&period=lifetime

{
  "data": [
    {
      "name": "post_video_views",
      "period": "lifetime",
      "values": [
        {
          "value": 0
        }
      ],
      ....

the page access token for page_b has same scopes as the one for page_a

I can see all the numbers in facebooks UI, but i cant not access it via the API, anybody had similar issues before?, im stuck and dont know where to go from here.

All above code have been ran in the Facebook Graph Explore, with newly generated tokens.

Any inputs on where to go from here are appreciated.