If you export JPG's from Export for Screen and even if you have the Art Optimized option selected, you're going to get the bad image quality from using hte "Type Optimized" feature regardless if you want it or not. I've updated everything to the latest versions and still no fix?
Sorry about this trouble. This has been reported by a few other users as well, but unfortunately we're unable to replicate this. I would request if you could share a few more details like:
OS and exact version of Illustrator
A sample .ai file along with exported .jpegs showing the difference. Screenshot of the export settings or you may share a small video of the workflow. You can upload the files to the Creative Cloud (https://helpx.adobe.com/in/creative-cloud/help/share.html) and share the download link.