It’s possible the issue is that when calling the SES endpoint the Request is using ISO8859-1 rather then UTF 8, then the web service would think you were sending it three characters. That seems the most likely issue (we will see this is the internal email contents are quite different) .
Wh you send your Explicit test - you arent sending al the HTML you are only sending part of it (the text part) so it’s possible that somwhere in all the rest of the HTML is a problem.