This is just a guess. The Mavic needs time to process the sensor data and package it into a DNG file, but it also needs memory to store the images as they are being processed. Memory is limited, and this eventually limits the capture rate.
A typical way to design the software would be a pipeline architecture, with a task to copy the sensor data to a queue, and another task to save each image as a DNG file. The size of the queue would have to be finite, so even though you may be able to take 30 images in a row quickly, the system would not able to maintain that pace indefinitely.
The mod times of the individual dng files on the SD card might indicate the actual rate at which DNG files are saved. My guess is if you re-created your test the DNG files are created more than 2 seconds apart, which would say that the pipeline was getting further and further behind.
If the sensor data task compresses the data, then the capacity of the queue would be higher, and the time required to write the jpg to SD would be less than for a DNG file.