I notice that in your model, the sample rate used is 6ms. Typically, the gap in output data is observed when the sample rate is less than 10ms because the target tries to upload the data to the host every few seconds.
To resolve the issue,
1) Please increase the duration further in your model and see if it satisfies your requirements. Ideally, it is not recommended to run external mode on Arduino targets at very high rates (such as 1 ms).
2) If your requirement is to have data archiving at very high speeds, it is recommended to use a serial block for data upload. You would then use an ICT receive block on the Host to log data into MATLAB.
I hope this addresses your query.