As others have said, you’re probably thinking of Tenet. I watched it and by the ending I was a bit confused but thought the point of the movie was to be confused on the first watch and the second watch will explain a lot more. NOPE. I watched it again within a week and nothing that is explained in the first half holds up in the second half. They explained the rules and then ignored them to push the plot along. Great idea, very bad execution though.

Yeah I find myself humming That Funny Feeling quite often.