This issue occurs when user enables "Internet Explorer mode" on the Microsoft Edge as the Edge act like a Internet Explorer for specific websites.
You can enable or disable the IE mode with the help of below steps.

--> Click on 3 dots at the extreme top right corner on the Edge browser(Settings and More)
--> Click on Settings
--> Click on the default browser in the settings.
--> In the option "Allow sites to be reloaded in Internet Explorer" select the value "Allow" in the dropdown.
--> "Restart" button will appear after you change the above value and then click on it.
--> After the browser restart, Click on 3 dots at the top right corner of the browser again, you will find the option "Reload in Internet Explorer mode" and this option will appear as greyed out.
--> Open the Tibco Nimbus web client in the Edge now. the option "Reload in Internet Explorer mode" will be enabled. Click on it.
--> Tibco Nimbus web client will reload and you can enable or disable IE mode with the same steps.

If you check at the left side of the address bar there is an IE symbol will appear and when you click on it will say "This page is opened in internet explorer mode".