I've never seen a MV lamp strobe at a fixed rate. My SBMV lamps sometimes strobes during a hot restrike, but randomly.

If it is on a CWA, what may have happened:
The starting discharge probe became effectively a rectifier, so passing current only in one direction. It could be caused by the material difference (the main electrode has a coat with low energy barrier to leave electrons free, the probe does not could be one mechanism)
It is then slowly charging the series capacitor in the ballast.
The arctube temperature/pressure seems to be just so it is not able to hold the arc across the main electrodes, but when nearly doubled by the rectification, it is able to break down across the main electrodes so cause the flash.
This flash then discharge the capacitor, so the voltage becomes too low again, until the capacitor gets charged again.
This cycle then repeats and the lam is flashing until it cools down enough so the ballast OCV without the addition of the charged capacitor becomes enough to sustain the arc and the lamp lights up normally again.

For this to happen, few things have to align (the way the stsrting discharge rectifies, the polarity sensitivity of the main arc,...), so it happens only sometimes.