clear_notification does not work properly for notification groups

I'm not sure if I should post this issue here or in the core repo, but as its specific to iOS I figured this would be a good place to start first.

**iOS device model, version and app version**

Model Name: iPhone 11p & iPhone XR
Software Version: 15beta & 14.7.1
App version: 2021.7 (2021.202)

**Home Assistant Core Version**

core-2021.8.0

**Describe the bug**

1. Created a notification group with our two iPhones (notify.all_iphones).
2. Created an automation which sends a notification (including a tag)
3. After x min timeout I want the notification to disappear on both iPhones using the clear_notification with tag

But for some reason the notification only disappears on one of the phones. The iPhone 11 Pro to be exact, the notification remains visible on the iPhone XR.

Notify group:
```
- platform: group
name: all_iphones
services:
- service: mobile_app_iphone_xr
- service: mobile_app_iphone_11_pro
```

Automation:
```
alias: Verwarming - Zet 's ochtends aan
description: ''
trigger:
- platform: time
at: '08:00:00'
condition:
- condition: state
entity_id: binary_sensor.someone_home
state: 'on'
- condition: template
value_template: >-
{{ not is_state_attr('climate.thermostaat', 'temperature',
states('input_number.temperature_high')) }}
action:
- service: script.set_high_temperature
- alias: Set up variables for the actions
variables:
notification_tag: '{{ ''TAG_'' ~ context.id }}'
- service: notify.all_iphones
data:
title: Verwarming
message: >-
Automatisch ingesteld op {{ state_attr('climate.thermostaat',
'temperature') }}℃
data:
tag: '{{ notification_tag }}'
- wait_template: ''
continue_on_timeout: true
timeout: '00:00:30'
- service: notify.all_iphones
data:
message: clear_notification
data:
tag: '{{ notification_tag }}'
mode: single
```