All HTML markup is stripped when you save an article

From Joomla! Documentation

In some cases (for example, when you have a database error during the update), when you edit an article, all of the HTML markup is removed when the article is saved.

To fix this, navigate to Global Configuration → Text Filters, make sure the filters are set correctly, then Save.

This is caused by moving the Text Filter function from Article ManagerOptions → Global Configuration. In a normal 2.5.0 update, these Text Filter settings are transferred automatically.