Add CSS to the "calcite-accordion-item" which will allow screen readers to recognize the header in the accordion. Adding role="heading" level="2" is a solution that works for our requirements but needs to be added individually to each accordion.
Example
<calcite-accordion-item expanded="true" heading="Past Events" role="heading" level="2" style="background-color:##E3ECF1; color:#ffffff; border-bottom:0px solid #a6a6a6">
Testing Tools: Apple iPhone 12 Mini with VoiceOver; Samsung Galaxy S9 with TalkBack; native mobile accessibility settings; Windows Desktop with Chrome Browser and Accessible Color Picker extension; Firefox Browser; standard QWERTY keyboard; NVDA Screen Reader; JAWS Screen Reader; aXe DevTools; ANDI Section 508 Accessibility Checker; iOS Desktop with Safari Browser with Voiceover enabled.