On my machine a copy and paste of text from a Web Document tab, usin the Control-C and Control-V keyboard commands, only copies the first line of text.
I can consistently replicate the problem with this procedure...
Create a new Web Document, enter the URL and description
Do not type anything in the Notes field at this time
Click on the Web Document tab and visit the web page
Copy some text from the web page using the Control-C keyboard command
Click on the Notes tab and use keyboard command Control-V to paste the text
Only the first line of text is pasted.
The problem does not occur if one selects Copy and Paste via the edit menu or by right clicking and using Copy and Paste in the context menu.
This does not appear to be a keyboard issue as keyboard commands for copy and paste work normally in all other apps on my machine.