Somewhere between version 18.2.59 and the latest 18.3.44 an extra <div> has been added to appointments if using an eventTemplate.
Previously, there was a div with class "e-appointment-details" which we could style and was inherited inside the next div, e.g. "template-wrap"
There is now an additional <div> between these so custom styles for e-appointment-details are not being inherited.
<div class="e-appointment-details">
<div>
<div class="template-wrap">
<div class="event-status" style="background-color: rgb(255, 255, 255);"></div>
<div class="event-detail"><div class="event-subject">2:40 - Mr Nicholas MacWilson</div>
<div class="event-description">Fillings</div>
</div>
</div>
</div>
These are also present on your event template demo...
https://ej2.syncfusion.com/react/demos/#/material/schedule/event-template