I can't think why someone wouldn't be worried about a flooding basement when the shower is turned on.

You need to determine if the water is due to a failed shower pan, a bad drain, or the water supply lines. Are you sure it's from the shower being turned on, or is it possible that it's from another source in the bathroom? To test the pan, you need to fill it from a source other than the shower itself. That means filling buckets from outside the bathroom and carefully pouring the water into the pan.

Is this shower directly above the basement? Do you have access to it from below?

Use your flashlight to peer up into the depths of the basement ceiling, and find out where the water is coming from. The valve? The drain? What is making it rain?

Pay attention to #1 and 2 for starters (since it passed flood test): https://cdn.ymaws.com/www.tile-assn.com/resource/resmgr/docs/help!_my_shower_is_leaking.pdf

Just because it passed the flood test doesn't mean it can't be the liner. If it's a surface-applied membrane, it could have been compromised after the test, or the water might not have been filled up high enough to fully test the pan. The corners at the curb might not have been done properly.

@catbuilder, True. Makes sense to eliminate the potential of a plumbing issue first.