Shiny: flexdashboard gauge disappears when changing theme

Question

The HTML gauge widget disappears from my flexdashboard after I apply some minor thematic changes, only when runtime is set to Shiny. Here is a set of minimal examples that illustrate the issue

This produces an empty output:

---
title: "Untitled"
runtime: shiny
output: 
  flexdashboard::flex_dashboard:
    orientation: rows
    theme:
      navbar-bg: "#b5b5b5"
---

library(flexdashboard)

gauge(5, min = 1, max = 10)

Image of emtpy output

As soon as I remove the theme argument from the top I get the gauge back

---
title: "Untitled"
runtime: shiny
output: 
  flexdashboard::flex_dashboard:
    orientation: rows
---

library(flexdashboard)

gauge(5, min = 1, max = 10)

Image of working gauge

If I remove the runtime: shiny part of the header I can theme my flexdashboard any way I want and the gauges do not disappear. Obviously if I do that I cannot add any Shiny elements to the dashboard anymore. Any ideas why this is happening?

Using the renderGauge() function does not help, nor does it make sense in this case as there is no reactive input to the gauge.