NewUpdateDraftActionResponseBuilder crashes the Gmail IOS app - google-apps-script

Question

On the current Gmail iPhone app, calling the newUpdateDraftActionResponseBuilder crashes the app. No errors get logged and those crashed calls never appear in the 'Executions' tab in the App Script console.
The same code runs fine on the Android Gmail app or in the browser.
Code to reproduce
index.gs
function attachRecording() {
return CardService.newUpdateDraftActionResponseBuilder()
.setUpdateDraftBodyAction(CardService.newUpdateDraftBodyAction()
.addUpdateContent('<h1>hello</h1>', CardService.ContentType.IMMUTABLE_HTML)
.setUpdateType(CardService.UpdateDraftBodyType.IN_PLACE_INSERT))
.build();
}
appsscript.json
{
"timeZone": "America/New_York",
"exceptionLogging": "STACKDRIVER",
"runtimeVersion": "V8",
"oauthScopes": [
"https://www.googleapis.com/auth/gmail.addons.current.action.compose",
"https://www.googleapis.com/auth/userinfo.email",
"https://www.googleapis.com/auth/script.external_request",
"https://www.googleapis.com/auth/gmail.addons.execute"
],
"addOns": {
"common": {
"name": "Vocal",
"logoUrl": "https://www.gstatic.com/images/icons/material/system/1x/mic_black_48dp.png",
"universalActions": [
{
"label": "Manage account",
"openLink": "https://app.vocal.email"
}
],
"homepageTrigger": {
"runFunction": "onWelcomeClick"
},
"openLinkUrlPrefixes": [
"*"
]
},
"gmail": {
"composeTrigger": {
"selectActions": [
{
"text": "Record voice message",
"runFunction": "attachRecording"
}
],
"draftAccess": "NONE"
}
}
}
}
I tried the different methods of the function listed here: https://developers.google.com/apps-script/reference/card-service/update-draft-action-response-builder
Whether it's adding a subject or text/html in the body. it always crashes the app.