Martingale is to double the lot size on loss. Loss means that the last closed trade is not profitable and it's also closed.
Some people wanted to do it in the "wrong" way - do calculate the lot size from the newest already running trade for example. So the block first searches for such trade and only if it doesn't exists it takes the lot size from the last closed trade.
For Martingale there is no memory where the last lot size is stored. It depends on the lot size of the last trade. If that trade was loss - it doubles that value, otherwise the value is reset to the initial. And of course it depends, because there are many options, but I'm talking for the default Martingale settings.
So I think that you have one trade still running when the new one is created and it works with its lot size. If you want to separate things, try with different Group numbers.