I would t worry… yet
I’ve kept Duncan’s as long as I’ve been in the hobby.
I’ve seen the tentacles super long and flowing in the breeze like the hair of a guy driving a convertible in a Levitra commercial.
I’ve also seen them bubble like an anemone, curl up like you’re seeing, and even completely close at seemingly random intervals.
I did recently lose a huge colony to brown jelly disease, which was really surprising as it had been indestructible for years.
keep an eye on it, but they do change their morphology a lot in my experience.