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That has happened to me a few times over the years. Sometimes, rebooting fixed the problem but, more time than not, there was a hard to find unread email (usually a reply chain) I had to track down.
 
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But I’ve never had this problem until iOS 26
Not sure then. It's been that way for years with me regardless of 2 devices or 6, badges never update unless I open the application on macOS or iOS. Pretty sure it's just the way Apple pulls emails from the server.
 
Also check if there are some in the Junk folders - can be new there and thus counted but no shows up on main screen
 
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