I've just switched off the auto mark as read and tested your case using a forward option and I agree with you. This is a bug.

I forwarded a new unread email to another of my accounts. I noticed: The new mail star icon remained on Inbox folder The new mail icon remained displaying on email. There was no 'forward' icon displaying.

I reported as bug - link below so you can follow. Bug report : https://bugzilla.mozilla.org/show_bug.cgi?id=1791272

However, until this gets fixed there is a way to disable the 'new mail' icon on folders and messages in thread pane.

I do not know if you have ever created a 'userChrome.css' file, so I'm going to provide full information.

• Menu app icon > Settings
• Select 'General'
• Scroll to the bottom and click on 'Config Editor' button
• Accept Dragons warning
• In search type: legacy
• Look for this line: toolkit.legacyUserProfileCustomizations.stylesheet

If it is set to 'False'

• Double click on that line or use toggle icon to toggle the 'False' to 'True'

In Thunderbird

• Menu icon > Help > More Troubleshooting Information
• Under 'Application Basics - 'Profile Folder' - Click on 'Open Folder'

A window should open showing the contents of your 'profile name' folder.

• Exit Thunderbird now.

• Create a new folder and call it chrome - note the spelling - all lower case.

The 'chrome' folder should be in the 'profile name' folder - same place as the 'Mail' folder. See image below.

• Open a text editor like Notepad.
• Copy everything between lines below into Notepad.

/*
 * Do not remove the @namespace line -- it's required for correct functioning
*/

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

/* Hides the 'new mail' icon in Thread Pane */
treechildren::-moz-tree-image(subjectCol, new) {
  list-style-image: none !important;
}


/*Hide the 'new mail' icon in Folder Pane */
#folderTree treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true) {
 background-image: none !important;
}

• save the file as userChrome.css (note spelling - all lower case except for the 'C' ) in the 'chrome' folder. See second image.

check you really have saved it as a userChrome.css (Cascading Style Sheet Document) and not a Text Document with filename userChrome.css.txt

Start Thunderbird.

Thank you Sir, I will try to apply this detailed solution :).