I've never had an alignment pin move, and don't remember hearing about it happening in an MP mold, but I think that's the problem I have been dealing with.
I started using a new 9mm mold from Miha. Perfect on inspection right out of the box and after initial mold prep. Great casts in the first two or three sessions, but then persistent finning and feathering started to appear, along with visible parting line defects, almost always from the boolits dropped from the far end of the mold.
A close look showed visible light between the closed blocks, wedge shaped, with contact only at the near end, a widening gap at the far end, with the only other contact at the far alignment pin. The male pin was proud, and the female pin nearly so, or at least not as inset as the other female pin. I pressed the blocks together with a hydraulic shop press (hugs from a vise and love taps with a hammer didn't budge the pins at all), applying pressure just over the affected pin set, with lead ingots protecting the blocks themselves, and the mold is back to dropping great boolits.
The only thing I can think of as a cause is that the mold got very hot on preheating prior to the problem appearing. I'd built an insulated mold garage that works too well: what used to need at near max setting with molds sitting a long time directly on the coils to get to casting temp (205° C, or 400° F, measured with a K probe set in one of the cavities) now takes a medium low setting and much less time, even with an aluminum pad between the blocks and coil. A buddy I'm showing how to cast started preheating while I was away doing something else, saw the temp reading rise to around 200°, and let it go up, mistaking Celsius for Fahrenheit. He got sidetracked, and I came back to find the probe reading 450° C (~840° F), and for an unknown time.
All seems OK for now. I've read here that Lee doesn't recommend letting their aluminum molds staying over 450° F for long because there can weakening or warping of the blocks. MP molds are a different alloy, I think. Anybody know for sure?