Hi thanks! I'll keep that in mind. Although all the files said they were encoded for UTF-8, your suggestion might catch other conversions happening further behind the scenes.
For my latest adventure in sending epub to Kindle:
Validated my epub file on
https://www.epubvalidation.com. It had a bunch of old tags in it that affected other tags. So I manually edited the ePub. It took some work and some research but I did it in a few hours (I'm slow). After getting all the errors resolved, it was finally processed by SendtoKindle.
Before doing all that, I tried reconverting the ePub from the original AZW3. Looks like I'll be digging deeper into options in the Converter tool.