Titles not showing in blog posts

Titles in individual blog post were showing OK, but suddenly they are not showing at all. I’ve inspected the code and it shows the following:

<header class="entry-header full-width-image no-title" data-url="https://www.example.com/whatever/">
</header>

So it appears like something is indicating that it has no title (“no-title” class) but every post has a title and it doesn’t show in any of them.
And I haven’t checked the box “Hide title” in any of them.

I have no idea what could be causing this. Any ideas?

Thanks!