I have found that a lot of the older MTS in my tanks will die when I move them/ redo everything. But usually, they bounce back as they don't all die. If there are other snails in the tank at all, they will outcompete the MTS because ramhorns and pond snails reproduce by laying egg sacks and MTS lay one egg at a time. Although they are all prolific breeders, the other snails are much more so. Also, I've found that Apple Snails are excellent scavengers, akin in their constant search for food to Goldfish. If there is food in the tank they will eat it until it is gone. The larger MTS were probably starved out, but eventually their food sources will come back and also the population. I had a similar problem in a 2.5 tank where I tried to seed it with MTS from my 30 gallon. I guess I missed some hiding ram horns because their population exploded which was what I was trying to avoid. I had hoped that by only adding MTS it would at least keep the population to something I like. It has worked somewhat before. It is 5 months since I started that tank and there is finally signs of tons of little MTS showing up. I think you experienced a similar situation.