I had this issue before and it was never resolved. I thought it was due to an error I was getting when upgrading my PHP version, but that was unrelated.
Currently, i’m on MODX 2.8.1 and PHP 7.3. I’m trying to update the RTE Extra from v1.2.1-pl to the current v1.4.0. But every time I do this, the content area of my resources just disappears. I’m not sure how to proceed. The Modx Error logs and server logs show nothing either. So I’m out of ideas.