When you update the title of the page, you can see updated title in the page properties but in the browser tab it shows older title name.
Sometimes you see update title name in page properties but on the browser tab title name is different and it is still old one.
you can add script editor web part on the page and following script:
You can inject below JS code to the modern page and then the browser tab title will get updated to the text you typed in the "Your Title Goes Here".
<script type="text/javascript">
document.title = "Your Title Goes Here";
</script>
expertbrains 
My modern pages are in the Site Pages library rather than Site Assets. Here’s a no-code solution that worked for me today
1. Locate the page (e.g., in the Site Pages library)
2. Click the ellipsis to the right of the file name, then click More > Properties
3. Edit the Title field to show the desired page title
4. Visit and refresh the updated page – the new title should appear
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