No TagKey: "<tag>" - User tags are ignored by cost explorer

I want to group the following usages by a tag named "organization":

• Client VPN endpoint association hours
• Client VPN endpoint connection hours
• EC2 instances hours

So I set the "organization" tag on the following resources

• Client VPN endpoint
• EC2 instances

I also activated the user-defined cost allocation tags in the Billing panel.

Although, when i group my resources using the "organization" tag, here is the output I get :

Seems like none of my resources have the "organization" tag defined, although it's absolutely not the case. For example, this is a shortened describe-vpn-endpoints output :

{
    "ClientVpnEndpoints": [
        {
            "Status": {
                "Code": "available"
            },
            "ClientCidrBlock": "10.1.0.0/16",
            "SplitTunnel": false,
            "VpnProtocol": "openvpn",
            "TransportProtocol": "udp",
            "VpnPort": 443,
            "Tags": [
                {
                    "Key": "app",
                    "Value": "xxxxx"
                },
                {
                    "Key": "organization",
                    "Value": "630c8acfd564a41dae9d73e1"
                },
                {
                    "Key": "Name",
                    "Value": "o-ABEFO_XXXXXXX-630c8acfd564a41dae9d73e1"
                }
            ]
        },
        {
            "Status": {
                "Code": "pending-associate"
            },
            "CreationTime": "2022-08-30T12:55:51",
            "ClientCidrBlock": "10.1.0.0/16",
            "SplitTunnel": true,
            "VpnProtocol": "openvpn",
            "TransportProtocol": "udp",
            "VpnPort": 443,
            "Tags": [
                {
                    "Key": "app",
                    "Value": "xxxxxx"
                },
                {
                    "Key": "organization",
                    "Value": "630e0787814fdcf162bd521f"
                },
                {
                    "Key": "Name",
                    "Value": "xxxxxx-o-flomo-630e0787814fdcf162bd521f"
                }
            ]
        }
    ]
}

What am I missing ?