Built-in kitchen wastewater installation

I bought an expensive fitted kitchen – it turned into a nightmare with 1001 problems. Here is just one. Two installers said (independently of each other) that the kitchen was wrongly planned. The flexible hose from the siphon goes upwards into the wall connection, so wastewater flows back. That would lead to 2 problems.

1. Wastewater could flow back into the washing machine and the dishwasher. The appliance wastewater hoses end at 49 cm height from the floor into the siphon, the flexible hose at 52 cm into the wall.

2. It could cause odor problems.

The installers said that with a higher plinth for the kitchen units and a shallower sink, the flexible hose would no longer run upwards to the wall. That way the problem could be fixed.

The company that planned it says the slope is within the standard range. There would be no problem at all; backflow and odor could not occur. Is that true? Is the installation really unproblematic? Does nothing have to be changed?

Thanks for an answer!