MikeMcQ - Thank you for that insight and the links to those tests. Very interesting!
I don't know where the last certificate in the chain came from. The first two are from Let's Encrypt. I don't know what Digital Signature Trust Co. is. I found a post here that references that issuer, but I'm quickly getting beyond my depth of understanding. But there are no custom root certificates installed on my NAS.
Any ideas how I fix this issue with the chain?
Fwiw, I did just post to the QNAP forums here.