inconsistent or randomly disappearing album art on an iPod Classic is usually caused by a corrupted iPod artwork cache database or image files that are too large for the iPod's ancient hardware to load. Because you use Mp3tag to embed the art directly into your MP3s, your source files are fine; the issue lies in how iTunes processes them onto the device.
Try this
Connect your iPod Classic to your computer and open iTunes.
Click on your iPod icon in the top-left area, then click on the Music tab under your device settings.
Uncheck "Display album artwork on your iPod".
Click Apply / Sync. This will strip the broken artwork cache completely from the iPod.
Re-check "Display album artwork on your iPod" and click Apply / Sync again. iTunes will rebuild the thumbnails from your Mp3tag embedded files from scratch.