As you can see I follow the tag "thermodynamics". And only a few questions are highlighted. Why?
I use: Chrome Version 76.0.3809.100 (Official Build) (64-bit)
This is a feature.
While on the normal question site all thermodynamics questions will show up highlighted, you have just searched for questions with thermodynamics (note it says
[thermodynamics] in the search bar on the top of the page). In the search view, only those questions are highlighted that have another tag you follow in addition to thermodynamics. If I had to guess, you also favourited kinetics which would be why the question on the top of the list is highlighted.
The reason behind this feature is that an all-yellow list would provide exactly zero information.