where did you add that CSS? Currently i can’t tell where as its being combined in an Autoptimize cache file – which means its being loaded first, then the Simple CSS containing the standard styles is being loaded which is overriding the mobile styles.
Move the styles to the Simple CSS and make sure it is below the existing .portfolio-caption styles.
Simple CSS is loaded before the Additional CSS – so additional CSS rules will overwrite the same rules in simple css. However when you combine CSS and load it as critical CSS using the cache plugin the order gets messed up.
I would advise putting all your CSS in only one of them.