EPS Carry lost zero (slightly) after a few hundred rounds

I have an EPS Carry on a CZ SP01 and when I went out to shoot it today it was off the the right by 2-3 inches at 20 yards. There was no slow loss of zero, just suddenly happened when I shot today. I had it torqued and loctited. When I went to check the tightness the screws had backed out maybe 1/4-1/2 turn, however it still felt extremely secure before tightening.

Is this out of the ordinary for pistol dots? I'm a bit surprised and disappointed but at least I know to check it more often now.