Hello
Apologies for your inconvenience.
Can you try installing Joomla core files again and clear the browser and Joomla cache?
You can also kindly check out the solutions to a similar problem in the following link. This will help you as well.
https://www.joomshaper.com/forum/question/5735
The reason can also be Advanced Template Manager. However, the problem is not only about the Advanced Template Manager. But in the case before the upgrade you have had multiple template styles linked to different pages. To solve the problem, you need to go to the template's menu binding settings, remove all selections, save, re-select, save. For the default template style - select everything, save, remove the selection, save.