There are two tests required for plumbing: the water intake and the drain/vent system. By plugging the future sink, shower and toilet intake pipes you separate the incoming water from the drain water.
The intake lines held without leaks but the drain pipes didn’t. Normally waters flows straight out of the drains into the septic system. But for inspection you have to fill the vent pipes from the roof, putting a massive amount of pressure on the drains.
So tomorrow more work needs to happen to prevent two pipes from leaking. Frustrating!