I fly into and out of the Schengen zone dozens of times each year for many years. A shrinking but still substantial proportion of my Schengen entries/exits do not get stamped (and I have no doubt that they didn't get stamped). Most of the Schengen zone passport control personnel either don't care to match my entry and exit stamps or they continue to give up looking to match my entries and exits (including for reasons related to non-stamping on various previous entries/exits). YMMV, but even the MUC, DUS, TXL, HAM and FRA passport control give up trying to match my entries and exits or even figuring out when I entered and exited.