In my experience, mother leaf last really long time. It slowly deteriorates, but usually after plantlet is quite big and roots secured. Photo below is just 1 example - see how big new plants are, and the mother leaves are sill attached?

I don't know what exactly is the reason, but maybe you are 'checking' to see if there are roots? Best is to leave them alone, they will grow at the speed they want to. Disturbing them is only slowing the process. I usually put the leaves on the surface, keep them dry for approx. 1-2 weeks or so. After I start watering just the same as the mature plant.