These used to work and now do not. I can't pinpoint exactly when they stopped working. I am using Capacitor 3 and upgraded from Capacitor 2 recently.
"@capacitor-community/fcm": "^2.0.2", "@capacitor/push-notifications": "^1.0.9"

This is the error I get:

Unhandled error { Error: The registration token is not a valid FCM registration token
    at FirebaseMessagingError.FirebaseError [as constructor] (/workspace/node_modules/firebase-admin/lib/utils/error.js:44:28)
    at FirebaseMessagingError.PrefixedFirebaseError [as constructor] (/workspace/node_modules/firebase-admin/lib/utils/error.js:90:28)
    at new FirebaseMessagingError (/workspace/node_modules/firebase-admin/lib/utils/error.js:256:16)
    at Function.FirebaseMessagingError.fromServerError (/workspace/node_modules/firebase-admin/lib/utils/error.js:289:16)
    at Object.createFirebaseError (/workspace/node_modules/firebase-admin/lib/messaging/messaging-errors-internal.js:35:47)
    at /workspace/node_modules/firebase-admin/lib/messaging/messaging-api-request-internal.js:79:51
    at process._tickCallback (internal/process/next_tick.js:68:7)
  errorInfo:
   { code: 'messaging/invalid-argument',
     message:
      'The registration token is not a valid FCM registration token' },
  codePrefix: 'messaging' }

Things I have tested:

1. Tried a message to the topic from the cloud function. Works.
2. Tried a message to the token from the cloud function. Did not work. See error above.
3. Tried a message to the topic from the FCM console. Worked.
4. Tried a message to the token from the FCM console. Did not work.
5. Checked the google-service.json file against a latest version from the firebase project. Looks fine,
6. Testing on iOS topic and token. Works fine.
7. In Play store Setup -> App Integrity the "Upload Key Certificate" SHA-256 matches the Firebase project for the app. But the "App Signing Key Certificate" SHA-256 does not match. Is this an issue?
   

Cloud function code:

const notification: admin.messaging.Notification = {
      title: title,
      body: body
  }

  const message: admin.messaging.Message = {
    notification,
    token,
    android:{
      notification:{
        sound: 'default',
        icon: 'push_logo',
        color: '#000000',
      }
    },
    apns:{
        payload:{
          aps: {
            sound: 'default'
          }
        }
    }
  }

  return admin.messaging().send(message)

Which creates:

{ notification: { title: 'test', body: 'test' },
  token:
   'eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJhcHBJZCI6IjE6MTAxMzMxMzU1NjY5NDphbmRyb2lkOmQyODI0NGY1MWIzYTkyYTMwN2Y5NzciLCJleHAiOjE2NTk3NTExNTAsImZpZCI6ImV3YkF0c1psUm5xZ2Mzb0tQRWs0VnYiLCJwcm9qZWN0TnVtYmVyIjoxMDEzMzEzNTU2Njk0fQ.AB2LPV8wRQIgbdIAgIU76ziJc84g5gcNFNzFyid2xeDTcAywjecKFKoCIQD1KkflpXmfOSvp28XVmTtm4JtWaaVcycQRMXtKSNUM0Q',
  android:
   { notification: { sound: 'default', icon: 'push_logo', color: '#000000' } },
  apns: { payload: { aps: [Object] } } }

Ionic (Angular) app code:

PushNotifications.requestPermissions().then((permission) => {
      if (permission.receive === 'granted') {
        PushNotifications.register().then(result => {
          this.subscribeToTopic(tenant.id);
          // Register for some topics here. Code removed.
          this.saveFCMToken();
        })
        .catch(err => {
          console.log('Push register error');
          console.log(err)
        });

 private async saveFCMToken() {
    try {
      const result = await FCM.getToken();
      const member = this.authService.getCurrentMember();
      if (member) {
        // Save the token against the member
        await this.authService.updateMember(member.id, { fcmToken: result.token });
      }
    } catch(err) {
      console.log('Error saving FCM token')
      console.log(err)
    }
  }

UPDATE I have found the problem although I have no idea why this happens. On Android if I use

import { FCM } from "@capacitor-community/fcm";
const result = await FCM.getToken();

it returns a token that DOES NOT WORK and looks like this: eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJhcHBJZCI6IjE6MTAxMzMxMzU1NjY5NDphbmRyb2lkOmQyODI0NGY1MWIzYTkyYTMwN2Y5NzciLCJleHAiOjE2NjM2NDI0NDIsImZpZCI6ImNhNk94R2lFU0dXYVk1RGU1UTBNTWIiLCJwcm9qZWN0TnVtYmVyIjoxMDEzMzEzNTU2Njk0fQ.AB2LPV8wRgIhAO4---Dns2vDbIdOZsamrVJ4TtEQhPK3qktfX-Q305E3AiEAo7DoZmfKha0LhDPy2QjQrbJBXyfopTnk5AgdZUsik3k
However if I use

PushNotifications.addListener('registration', (token: Token) => {
      console.log('token.value', token.value)
    });

it returns a token THAT WORKS and looks like: ca6OxGiESGWaY5De5Q0MMb:APA91bFxBvkS_qlMo0DW9F9nVeRsffM5Llms2DZKeq_SfGJBFSyN63LjBQh_CYW7qpFQeTJnjJmDOW1eaIiOH0GTZbxktWkfj_IYrrtdwzwbGVidOEbn67KR7508Uf0kFJKg6vlVeTey
On iOS await FCM.getToken() works fine.

UPDATE 2 Ah found the issue here: https://github.com/capacitor-community/fcm/issues/99