Firebase Messaging not Working in Modern Android Instant App - DisplayNotificationRequired?

I've read all the related SO questions, most of which were asked in answered in 2017 or early 2018, before Google simplified the way Instant Apps could be created. In my case, I created an "instant enabled app bundle" (described here) that works both as an app and as an instant app.

The app bundle includes a library I wrote that is configured to receive Firebase messages (described here) from the AWS Simple Notification Service (SNS). The problem is that messages are received when the app is run, but not received when the instant app is run.

The good news is that when I look at the AWS CloudWatch console, I can see every failed attempt. Here is the relevant part of the message:

"providerResponse": "{\"results\":[{\"error\":\"InvalidParameters: DisplayNotificationRequired\"}],\"multicast_id\":\"8198293557962051\",\"success\":0,\"failure\":1,\"canonical_ids\":0}"

The message content is:

{
"to" : "fi_Pclw7RrWtPm0xMVSgbC:APA91bGJFzM6RQVisO0N_JOAb8rUOKBVPZ0I5jh9Vf-4f-xXtbQY_Ik7q3wLGeCbR5bh_lFWDy0PX-F2mIlamMlCTIuEqEOlk0KcFO9a5fYk6B_omGqevjY6KNiByI5j_vKQaF17Rif8",
 "data" : {
     "body" : "Content message",
     "title": "the Title",  
     "key_1" : "Value for key_1",
     "key_2" : "Value for key_2"
     }
}

I've searched for the DisplayNotificationRequired error but can't find anything. Anyone know what this means and how to fix? Thanks!

Since I wrote the above, I tried adding a notification object as well:

{
"to" : "fi_Pclw7RrWtPm0xMVSgbC:APA91bGJFzM6RQVisO0N_JOAb8rUOKBVPZ0I5jh9Vf-4f-xXtbQY_Ik7q3wLGeCbR5bh_lFWDy0PX-F2mIlamMlCTIuEqEOlk0KcFO9a5fYk6B_omGqevjY6KNiByI5j_vKQaF17Rif8",
    "notification" : {
     "body" : "Content message",
     "title": "the Title"
     },
 "data" : {
     "body" : "Content message",
     "title": "the Title",  
     "key_1" : "Value for key_1",
     "key_2" : "Value for key_2"
     }
}

Now I'm getting an error with MissingDataUri instead of a DisplayNotificationRequired error. When I use Postman to send this message directly to the device token, I also get a MissingDataUri error, so I'm thinking this is not a AWS SNS or a Firebase Cloud Messaging issue, but just an instant app issue.

Finally, I understand that there was an "instant app notifications beta" way back in 2018 that appears to be still running: https://g.co/instantapps/notifications

Is it still not possible to send a push notification to an instant app, specifically with a 'data' payload? (I went ahead an submitted the form...just in case.)

Thanks all!