This is one of the things I don't like about stacking tray systems. Worms will explore. You will get some worms crawling down into the bottom. There really isn't anything you can do about it. Some people will put a piece of landscape fabric between the lowest working tray and the bottom to catch strays.

Worms can get underneath the newspaper/cardboard lining and eventually into the leachete tray. I used landscape fabric underneath my bottom working tray, making sure the edges are pulled tight on all 4 sides.

Yeah, I found the cardboard did not prevent the worms from making their way down to the bottom tray. However, my worms seem to have decided to not visit there anymore. I haven't seen them down there for the last couple of weeks.