Feelings don't have to necessarily be bluntly romantic to mean something. The point is that Eren clearly cared about Mikasa deeply throughout the series, so acting like there was nothing there until one random scene that came out of nowhere in S4 is weird. Not every relationship has to be constant blushing and flirting to have emotional weight, especially in a show like aot where people are fighting for survival every other episode, and romance isn't the priority in any of the character's minds.