You are not doing anything wrong, the light is just moving under you. Pre-sunrise gains roughly a stop every ten to twelve minutes here, so a 10s frame metered at the start is already wrong by the time it finishes. What works: meter, then deliberately underexpose by about a stop, and shorten as you go. My Bedok sequence usually runs 15s, then 10s, then 6s, then 4s over about twenty minutes at f/11 ISO 100. Same filter throughout.