css – Published page versus preview of the same page

I have a classic SharePoint Online site where the logo of my designated site does not appear when I browse normally (there is only a blank space where it should be). However, the logo make It appears in the different configuration pages of the site, and also when I edit the page and see the "Preview" mode of the same page.

The site, which I inherited, has custom page designs and CSS and I think that's where my problem is, although I find it hard to reduce it, since everything looks good on those configuration / preview pages.

Are there any particular site / master page / CSS / ASPX settings that I need to see? Thanks in advance.