protected $fillable/protected $date not updating

I have a weird one happening. I have two fields, one of which ('response') is getting updated, the other ('resp_received') is not. I've tried both of the following in my controller and get the same result -- 'response' gets set but 'resp_received' does not.

Option 1:

// Response received datestamp
if ($request->update_type === 'response_received_datestamp') {
    $notice->resp_received = $request->resp_received;
    $notice->response = $request->response;

    return back()->with('success', 'Response updated!');
}

Option 2:

// Response received datestamp
if ($request->update_type === 'response_received_datestamp') {
    $notice->update([
        'resp_received' => $request->resp_received,
        'response' => $request->response
    ]);

    return back()->with('success', 'Response updated!');
}

In my model:

protected $fillable = [
    ...,
    'response',
    'resp_received',
    ...
    ];
protected $dates = [
    ...
    'resp_received',
    ...
    ];

And the form from my blade file:

<div class="modal fade row" id="response_received_datestamp_{{ $notice->id }}" tabindex="-1" role="dialog" aria-hidden="true">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <h4 class="modal-title">Set Response Received</h4>
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
            </div>
            <form method="POST" action="{{ route('notices.update', $notice) }}">
                @csrf
                @method('PATCH')
                <div class="modal-body">
                    <input type="hidden" value="response_received_datestamp" name="update_type">
                    <div class="form-group">
                        <label for="resp_received">Response Received</label>
                        <input type="date" id="resp_received" name="resp_received" class="form-control" value="{{ \Carbon\Carbon::now()->format('Y-m-d') }}">
                    </div>
                    <label for="response">Enter Response</label>
                    <textarea class="form-control" id="response" name="response"></textarea>
                </div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-default" data-dismiss="modal">Cancel</button>
                    <button type="submit" class="btn btn-success">Set Response Received</button>
                </div>
            </form>
        </div>
    </div>
</div>

Any thoughts would be greatly appreciated.