Please get in touch with us at any of the other support channels – https://helpx.adobe.com/support.html . Since this is not a generic issue that we can reproduce at our end, we will need someone to look into your machine to figure out what is going on here.
Ashutosh Chaturvedi | Sr. Quality Engineering Manager – Illustrator
An error occurred while saving the commentLiza Brown commented
This is very odd. I had this problem 4 years ago when I was running Illustrator 2016 on an older mac of that time, I am now having the very same problem on my very nice iMac Pro (Mojave OS) with the latest CC 2021 Illustrator: Copy and Pasting objects into a new or the same document randomly doesn't work and just gives me the flattened text from that complex object. (flattened in this case meaning, no longer on a curve)
I'm not using any auxiliary program such as Team Viewer. I was shocked when this happened as I assumed it was a thing of the past. I reset preferences, tried both keyboard, and menu copy and paste, checked "remember paste layers", all of it!
the only thing that worked was dragging instead of copy and paste (luckily you can drag from one open file into another, I never knew this or had forgotten)