Unable to change email notification text on booking edit page

I have an issue with changing an email notification text on sln_booking post type post edit page. When I make any change in a textarea field and then click ‘Send’ button, the user receives a message with an old version of the text. When I click ‘Update booking’ button after changing the text it does not update at all and the textarea still contains the previous version. Is there anything I’m missing out in this case or is it a bug?

Screenshot of the textarea:

https://ibb.co/L0BmZBc