Textarea typing bug when copy-pasting, hitting enter key, and typing

Demo video with key casting:

Approximate steps to reproduce: (it's not consistent, and I'm thinking there is some other step I've missed)

Enter an HTML textarea (I've confirmed this happens in both the GitHub and GitLab comment components)
Type some text.
Copy said text.
Paste the text in the middle of your existing text.
Click to the end of a line in the middle of your lines of text.
Type some letters like "asdf"
Hit enter several times. Notice that no newlines appear.
Type some more text like "test". Notice that the newlines you added in step 7 now all show up at once behind your previous cursor location.

I've got something similar happening to me on GitLab but I've also seen it happen on this website

Sometimes, just trying to type into a field does not work, and it takes a couple newlines for it to start working again.

Compatibility mode does not seem to change anything about this.