NewClipboardData system variable is set when MEW detects some other app changing Windows Clipboard content. The detection seems correct but can be easily broken by other apps. There are rules how messages have to be passed and if some app tries to be clever and doesn’t follow rules exactly as documented, MEW may not be notified. That’s why apps which blindly copy Windows Clipboard content work but MEW doesn’t. Paste Special performs such blind copy and fixes problem. However, this is hardly MEW problem and ClipMate developer’s reply seems to confirm it. So if anybody wants to use tools which play games with clipboard, it is necessary to learn how to use both Paste Special and internal MEW Paste.
BTW, clipboard change detection may depend on the order in which apps are started.