Hello!
Thanks for Your time.
Theme is: https://themeforest.net/item/handel-responsive-multipurpose-business-theme/20506941
And the category widget is made in: Appereance-> Widgets->Navigation menu.
This widget has been working fine from 2018 with portfolio pages. And it is still working fine, with old subcategories – there is no problem, just with the new ones. That is why I thought that maybe this could be more of a WordPress problem, not themes.. :/