Yes, it is a shame. That is due to using the Let's Encrypt "short chain". If you were using Apache or nginx, two more popular servers, this would not be a problem. By default those easily use the Let's Encrypt "long chain" which works for older Android. Sadly, your use of IIS server and the need to support older Android is more difficult with Let's Encrypt (see my prior post). IIS makes it difficult to set the Let's Encrypt "long chain".
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