An inherent issue with front load washers, and perhaps top loaders as well, is that some soapy water remains in the pump after the wash cycle and before the rinse cycle. The soapy water then gets mixed with the rinse water. It's more of an issue with front loaders because they use so much less water. Since some water always remains at the pump, the result is that the clothes are not fully rinsed of soap, even with a second rinse cycle.
I use the minimum amount of soap, the lowest marker on the cap of the laundry detergent.
I emptied the pump using the drain tube located in a panel on the front of the washer and took a photo.
Below sequential images of the water in the pump 1) after the wash cycle, 2) after the rinse cycle, and 3) after the second rinse cycle.
The image after the wash cycle, before the start of the rinse cycle, shows a strong content of soap despite using the first fill line in the detergent cup.
The water left in the pump will mix with incoming water at the start of a rinse cycle, so my having removed this water decreased the amount of soap. However, you can see that after 2 rinse cycles, the water is still a light shade of blue.
A hypoallergenic detergent may be better at rinsing off of clothes. You can also use less than the minimum.
If your clothes have stains, it's better to treat them before the wash than adding more soap to the wash. Treat them either by soaking or by applying stain remover directly to the stain and letting dry.
Increasing the default level of the water will help in rinsing and, subjectively, in washing performance, as mentioned in the following how-to video.
I became curious about raising the water level because I expected clothes to be tumbling in and out of water. Instead I could barely make out any water level. It was like watching tumbleweeds in the Arizona desert.
I increased the default water level for my machine. The result below from having used the first fill line of the detergent cup, and without emptying the pump after the wash nor after the first rinse cycle. The condition of the water left at the pump after the machine finishes is pretty clear. Not bad!
The higher default water level in the machine dilutes the soap enough to have a decent final rinse.
One more advice from Delia Ann Jones on Quora: Vinegar in the rinse water gets rid of excess soap suds.
Many washers have a tub clean cycle. DIY sites recommend adding white vinegar, baking soda, and an essential oil to the washer for the tub clean cycle. Do not add detergent. Google these sites for measurements. I'm not sure if this is better at cleaning the tub than what companies market for this purpose, but the ingredients are healthier.
I have the drain routed underneath the washing machine, as the machine sits above an extra sink I don't need to use. However, specifications for the washing machine, or general plumbing, require the drain outlet at a certain height relative to the washer, generally well above the washer drum water level. This prevents water from flowing down the drain when filling the washer.
Although the animation below has nothing to do with a washing machine, the siphoning action when the water is flowing from the right tank to the left tank is the same, where the left side is the drain outlet and the right side is the washing machine drum.
It's the same even though water drops from the end of the drain hose into the standing drain pipe, or in my case, the sink drain.
At the moment, my washer has no siphon break, but no siphoning occurs when the washer fills, because the drain hose curves along the top of the washer, well above the water level in the drum. (Nothing to do with this, but I think there may be a one way check valve so that any water left in the drain doesn't flow back to the drum.)
Some siphoning action may be triggered when the pump actuates water flow. However, I don't know how and if this could be harmful. Who cares if the pump doesn't need to work hard once the water starts siphoning itself down the drain?
I don't have an answer. At the moment everything seems to work fine.