DATETIME field is not cast to Carbon date correctly (missing time)

I'm stuck with this one:

I have a database that contains a DATETIME field, also called 'datetime'. I have the filed cast to a date via the model.

If I do a $foobar->getAttributes() I get:

Array
(
    [id] => 123
    [datetime] => 2018-02-10 12:34:56
    [created_at] => 2018-02-10 14:26:50
    [updated_at] => 2018-02-10 14:26:50
)

That seems great, as that matches what I see in the database.

However, when I output the Carbon date object $foobar->datetime, I get:

Illuminate\Support\Carbon Object
(
    [date] => 2018-02-10 00:00:00.000000
    [timezone_type] => 3
    [timezone] => UTC
)

And subsequently $foobar->datetime->toDateTimeString() outputs:

2018-02-10 00:00:00

So as you can see, the 'time' portion of the datetime value (12:34:56) is somehow lost when casting it to the Carbon instance.

Anybody got a clue why this is?