Empty subscriber list while channel subscribers count isn't zero - youtube-api

Question

Summary
I'm trying to get a list of channel subscribers, but sometimes I get empty response for specific channels with subscriber count > 0.
Details
Following this YouTube Data API documentation,
I'm trying to receive a list of the channel subscriptions, however did received empty list. This is not reproducible for all channels, but it is this channel -- UCWjFX5qjtUCF4Xr-Z4q0h9w.
I'm using this command to get the current subscribers:
curl \
'https://youtube.googleapis.com/youtube/v3/channels?part=snippet%2CcontentDetails%2Cstatistics&key=XXXXX&mine=true' \
--header 'Authorization: Bearer XXXXXX' \
--header 'Accept: application/json' \
--compressed
The API response is:
{
"kind": "youtube#channelListResponse",
"etag": "XXXXpU",
"pageInfo": {
"totalResults": 1,
"resultsPerPage": 5
},
"items": [
{
"kind": "youtube#channel",
"etag": "XXXXXXXaU",
"id": "UCWjFX5qjtUCF4Xr-Z4q0h9w",
"snippet": {....},
"contentDetails": {..},
"statistics": {
...
"subscriberCount": "5",
}
}
]
}
and then try to list the subscriptions:
curl \
'https://youtube.googleapis.com/youtube/v3/subscriptions?part=snippet,id,subscriberSnippet,contentDetails&key=XXXXXX&mine=true' \
--header 'Authorization: Bearer XXXXXX' \
--header 'Accept: application/json' \
--compressed
with the JSON response:
{
"kind": "youtube#SubscriptionListResponse",
"etag": "XXXXX3i0vIY",
"pageInfo": {
"totalResults": 0,
"resultsPerPage": 5
},
"items": []
}

Answer 1

Update the privacy settings in channel settings.

Answer 2

That happens because when invoking the Subscriptions.list with the request parameter mine set to true, you'll obtain a number of channel to which you -- that channel owner -- have subscribed:
mine (boolean)
This parameter can only be used in a properly authorized request. Set this parameter's value to true to retrieve a feed of the authenticated user's subscriptions.
But a channel's statistics.subscriberCount is counting the number of other YouTube users that subscribed to the respective channel:
statistics.subscriberCount (unsigned long)
The number of subscribers that the channel has. This value is rounded down to three significant figures. Please see the Revision History or the YouTube Help Center for more details about how subscriber counts are rounded.
For the other way around -- i.e. for a list of channels that are subscribers of your channel -- there are two other request parameters: myRecentSubscribers and mySubscribers.