Samsung and LG running Android 8, the app icon shows incorrect badge count

When Textra gets updated in Google Play (new release) the count showing on the app icon (e.g. 3) at that moment in time causes a persistent caching issue, resulting in the app icon badge count being incorrect going forward. It's a bug in the Samsung badger, but due to particular circumstances (Textra groups notifications) you don't notice the issue on other apps.

So to fix it, you need to mark all messages as read (Textra > top right dotdotdot > Mark all as Read) then uninstall Textra, install Textra again and you'll see the app icon badge then works as expected. 

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