There appears to be an incompatibility between the Pen theme and the popular Simple Calendar plugin. The issue is visible on the calendar schedule on site mockup.udance.com.au using the Pen theme. Summary details are missing when hovering over a calendar entry.
Google Chrome version 86.0.4240.75 on both a Windows 10 desktop and Pixel 3 XL phone were used for the test below. The test was also repeated with Microsoft Edge version 86.0.622.38 and Mozilla FireFox version 81.0.2 on the Windows 10 desktop. The result was the same. This effectively excludes the browser as being the source of the error.
Desktop browser view of the calendar entry on Oct 15:
Mobile phone browser view of the calendar entry on Oct. 15:
Now the same test is repeated after switching to another theme (the HoneyPress theme was chosen for no particular reason).
Desktop browser view of the calendar entry on Oct. 15:
Mobile phone browser view of the calendar entry on Oct 15:
Refer to the Pen support thread Anomaly observed with the Simple Calendar plugin for a workaround. It appears the Calendar plugin may require a tweak.