The issue is indeed with the thumbnail. Try going to Manage | Containers and disabling thumbnails. Does that result in higher quality output?
Unfortunately, this is a rather extreme fix for the issue as it affects all container fields in your file.
It's still a pretty extreme workaround, but maybe a file with an external data reference to your table(s) could be added where thumbnails have been disabled. Then use layouts in this file for printing/saving as PDF when the layout includes a container field that you need to be at best possible resolution. Scripts can automate the switch back and forth between files and layouts.