re: Plumbing Drain Lines not Sloped

So maybe not the perfect thread for this question but close enough.

In my second bathroom (humble brag), the water in the tub will drain only to the point that it's below the drain cover. So if I take the plug off, I can see the water sitting there about an inch down.

I live on the fourth floor of a five story condo building, and the master bath does not have the same issue. Is there any way to know what's likely causing this? It's not blocked because all the water drains and I've gone at it with one of those 3 foot long clog grabber things. Would be interested to know if it was faulty plumbing from the start, something within the walls of my condo, or something further down the line out of my reach.