Hey Csaba,
1). I’ve checked the situation and it seems that you’ve a third-party plugin installed that does this. The “Mav Menu Images” plugin replaced the project titles in H1 tags with project thumbnails – not only in the menu, as it should, but also in the content, for some reason.
2). The function accepts taxonomy slugs, which is why in this case the second argument should be “tag-portfolio”.
Sincerely,
Andrii / Satori Studio