On version 4.0.12 10777 with Joomla! 3.10.11.
Paste text and after clicking "Insert" it does not add the text.
Where are you pasting this text?
Here:
CTRL+C then CTRL+V works fine for me with TinyMCE and JCE Pro editors.
An insert problem has been noticed for JCE2.9.34
https://www.joomlacontenteditor.net/support/forum/113251-jce-pro-2-9-34-paste-plain-text-issue
Thanks. That is the issue with that version. Seems it gets worse after every update.