Mentioning of a user is split in first and lastname

I just discovered that from somewhere in December I do not get a mentioned user in a message as one mention any longer but splitted into two mentions with first and lastname. Is this a bug or is it by intention? 

This is how the JSON message to the bot for the message:  
@bot Sarah Johnson mentioning 

looked like until December:

 {
    "text": "<at>bot</at> <at>Sara Johnson</at> mentioning",
    "textFormat": "plain",
   ...
    "recipient": {
        "id": "...",
        "name": "bot"
    },
    "entities": [
        {
            "mentioned": {
                "id": "xxx",
                "name": "bot"
            },
            "text": "<at>bot</at>",
            "type": "mention"
        },
        {
            "mentioned": {
                "id": "123",
                "name": "Sarah Johnson"
            },
            "text": "<at>Sarah Johnson</at>",
            "type": "mention"
        },
     ...
    ],
   ...
}  

 And this is how it looks today. 

 {
    "text": "<at>bot</at> <at>Sara</at> <at>Johnson</at> mentioning",
    "textFormat": "plain",
   ...
    "recipient": {
        "id": "...",
        "name": "bot"
    },
    "entities": [
        {
            "mentioned": {
                "id": "xxx",
                "name": "bot"
            },
            "text": "<at>bot</at>",
            "type": "mention"
        },
        {
            "mentioned": {
                "id": "123",
                "name": "Sarah"
            },
            "text": "<at>Sarah</at>",
            "type": "mention"
        },
      {
            "mentioned": {
                "id": "123",
                "name": "Johnson"
            },
            "text": "<at>Johnson</at>",
            "type": "mention"
        },
     ...
    ],
   ...
}  

The mentioning is split in first and lastname with the same userid. Is that the intended behaviour?