It doesn't happen that the same message sometimes opens with images, and sometimes not. It is different messages, some open with images, some don't.
Also, it doesn't vary with connection WiFi vs mobile data. If the same message opens without images over WiFi, it will also open without images with mobile data. The same goes for messages with images.
Also, I tried other email apps, like Bluemail, alto, gmail etc. All messages with embedded images show them. But I would prefer to use your awesome app. I was using it for years on my old phone, galaxy s4, I really like it. Thank you!