Thanks for posting this issue, and welcome to the Prismic community!
This is a known issue, and our dev team has spent some time investigating a fix. It's something that we will fix eventually, but I can't say when.
Instead of managing all kinds of elements in the Rich Text field, we're increasingly encouraging users to use Slices to build pages. If you use an Image field in a Slice, the alt text copy-paste should work as you expect it to. (I know that it might not be possible for you to switch your project structure to Slices at this point, but I thought I'd mention it in case you're interested.)
I hope that clarifies things. Please let me know if you have any questions!