ᐅ Drainpipe with slope – is this acceptable?

After about nine months, I accidentally discovered the following:
A plumbing company installed a drain pipe in my new bathroom with a slight slope over a length of 1.20 m (4 feet).
Why can I judge this so well? The pipes (from the toilet and shower) were installed from the bathroom down to the room below (the heating room), where they run along the ceiling to the main sewer pipe. It was this section on the ceiling that looked suspicious to me, so I held a spirit level against it – there was a slope.
Now the question is: Is this poor workmanship, or could it be legitimate or even common practice?

In my opinion, waste doesn’t flow uphill... So, it’s not common practice. But based on what you’ve described, it shouldn’t be a problem to adjust the clamps on the ceiling along the threaded rod so that it slopes slightly downward, right?

Whether it is as simple as @seat88 assumes is hard to say.

On one hand, it is not clear how close the pipe is to the ceiling, and on the other hand, if the clamps can be adjusted accordingly, what does the area look like where the pipe passes through the ceiling into the bathroom? Is there enough space there?

However, if I read that this was done only recently, I would call the contractor back and ask them to fix it properly—that’s their responsibility.

They should also be aware that water rarely flows uphill willingly.

Or you need to put more pressure on the goal.

schxxx cannot flow upwards....

... has worked wonderfully so far?