Hi Drek,
Thanks for your reply and sorry for the late response.
Yes I am using latest version of IE and Edge, The issue exist due to the used font "Avenue" and is not exist on AGO version of the dashboard , but exist on the portal for ArcGIS 10.7
I was able to fix the issue by using different font and update this in the dashboard "app-dark-xx.css" and "app-light-xx.css" CSS files
I think the best solution could be, download the latest avenue font (used on AGO) and replace the on premise portal font with the AGO version.