Water Idle in closet bend

I am finishing a bathroom in a new home in the basement. Before installing the flange, I looked down the drain and noticed a small pool of water sitting idle in the closet bend (where the 3 inch PVC did a 90 to the main stack). I asked the homebuilder who said this was normal.

Brief history: When I first put down the toilet, the flush would produce air bubbles. The homebuilder came out and realized that the drain was not graded properly. So they pulled the drain up and ensured a proper grade.

Now I am about to install the toilet again and want to make sure that small pool of water is normal. Thanks!

A picture would help, but to me, if you see a "small pool" of water at the base of the closet ell, you have a drain problem....possibly no slope to slight back slope. Could be a big deal, but is just might be a situation which will never actually cause problems. For peace of mind, you might have that line camera'd to see just what the situation is.