hovertemplate/texttemplate/tickformat silently ignore a valid d3-format spec that begins with a sign flag

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/plotly.js/3.1.1/plotly.min.js"></script>
<div id="chart" class="chart"></div>

CSS

.chart {
  width: 700px;
  height: 260px;
}

JavaScript

var delta = 12.3456789;
Plotly.newPlot('chart', [{
  type: 'bar', orientation: 'h',
  y: ['example'], x: [delta],
  customdata: [[delta, delta]],
  hovertemplate:
    'A (:.2f)  = %{customdata[0]:.2f}<br>' +
    'B (:+.2f) = %{customdata[1]:+.2f}<extra></extra>'
}]);
// Expected hover:  A = 12.35    B = +12.35
// Actual   hover:  A = 12.35    B = 12.3456789