I’ll have to confer with others before I can send an e-mail and might need to redact it before sending.
But to be honest, the pattern is pretty clear on multiple devices with many e-mails. I’m pretty sure you or @eboehnisch can find them yourself after setting the iOS device to dark mode (and presumably focus on e-mail with HTML content). It appears to be dark mode that somehow changes the text to be black on black I think. The text is still selectable and becomes readable again when I set the device to light mode.
I think dark mode can be a tough cookie when it comes to external content you obviously don’t control. This issue appears to be similar to webpages or PDFs that don’t render properly. Hence my solution would be create an option to disable dark mode for the e-mail viewer like you can for PDFs.
I’m quite surprised not many other users reported this issue, unless of course it’s not a bug but some error in (many of) my archived e-mails. Or perhaps many of your users don’t use iOS dark mode?
Another solution might be to create an option for dark mode in DTTG all together and not follow device settings. I actually prefer apps that allow me to choose between manual dark mode or automatic dark mode, as more apps have difficulties when using dark mode. As said, specifically when it comes to external content. The PDF option in DTTG creates a nice hybrid betweem them.