Not sure why the following code is not returning the Event's subject when exactly the same code, but replacing Event for Contact, is working without any issue. This component is executed as a Quick Action on the Event page, and due to it is not possible to use LWC on this object, the LWC is injected inside an Aura component, maybe this is the reason?
Aura component
<aura:component description="Modify Appointment"
implements="force:hasRecordId,force:lightningQuickAction"
controller="AppointmentService">
<c:editAppointment recordId="{!v.recordId}"/>
</aura:component>
LWC html
<template>
<template if:true={event.data}>
<div class="slds-var-m-around_medium">
<p>{subject}</p>
</div>
</template>
</template>
LWC js
import { LightningElement, api, wire } from 'lwc';
import { getRecord } from 'lightning/uiRecordApi';
const FIELDS = [
'Event.Subject'
];
export default class EditAppointment extends LightningElement {
@api recordId;
@wire(getRecord, { recordId: '$recordId', fields: FIELDS })
event;
get subject() {
return this.event.data.fields.Subject.value;
}
}