“Heart rate is not a reliable way to monitor intensity during short efforts at high intensity because heart rate lags behind abrupt changes in output.” See here: https://www.8020endurance.com/intensity-guidelines-for-cycling/
That’s one of the drawbacks of using HR as a training target. Power or perceived exertion would be better for intervals.
Rprego is correct with stating that HR lags so for shorter intervals it will be behind the effort you producing. RPE is your next best guide if you don’t have a power meter. Review you workout, and the zones and their descriptions and then use your perception to determine how hard you are working. Don’t think this is not as powerful as having gadgets, learning how to feel your efforts and then cross checking with the gadgets is your most powerful metric tool!