I just quickly ran this and for me it looks good. Probably it is the issue described by @Colin that it is not a realtime environment and if your pi is busy with other things it might become irregular.
Calculated the difference between consecutive timestamps and it looks like this:
You can see that no diff = 0 was recorded (blue numbers for the counter nodes).
The burst function from here (by @JGKK), slightly modified, but very useful for testing:
demo flow (edited, to clear chart before each run):
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