This is not relevant to xfce4-terminal, but to bash (which I assume you’re using). In version 5.1 of bash the setting enable-bracketed-paste is set to on by default. This feature will highlight the pasted text in the terminal.
To disable it, close your terminal, add this line in your ~/.bashrc file
bind 'set enable-bracketed-paste off'
and restart the terminal.
For more information you can see the new features of bash here or read man bash.