Water flowing back to Washing Machine

[bwspot]

I do live in the condo that uses one drainage pipe for sink, dish washer and washing machine.
Many times i see water in washing machine.
The drain pipe from washer goes up and has a loop already.
If i open the cleaning opening and pour some water i can see water immediately draining via the cleaning port instead of going back to main drainage pipe.
Should the drainage pipes be at angle to force the water go into the main drainage pipe direction?
I had my drainage pipe cleaned few times by plumbers via the Shnaking/Cabling method but the issues comes back and even happens just after the pipe was cleaned.
What is strange that i can forcefully pour out tons of water into the sink to test and nothing comes back the washing machine.
Then during cooking prep and cleaning dishes in the sink (NOT washing machine which i barely use) water will many times end up in the washing machine.
Cannot find exact pattern when it happens.
I wonder if the pump in the washing machine has the CHECK VALVE.

I found that the pump is this one but i dont know if it has check valve
https://www.searspartsdirect.com/product/2ik0lrn6s3-0026-417/id-132611913
there is some housing the pump is attached to and i dont see any check valves.

I wonder if installing the one way flow valve could be a solution or i should redo the connection and
make sure the main drain pipe is at the slope and maybe separate the sink from the washer drain

Any advice from the pros what would be the best solution?
Attached some photos of how the pipes are connected.

 

[bwspot]

i did some testing and removed the washer pipe and when i poured water over right side of the sink it comes out of the washer pipe connector instead of draining via main drain pipe. When i pour water over left side nothing comes out. Seems like that setup is wrong and sink should not be connected like this with washer drain. I found old photos how this was originally setup when it was working and it makes much more sense as sink was separate from washer.

Still from time to time i still see water coming back when i only use left side of the sink.
So i would like to understand how this works.
Is it because drain for the washer is slightly angled down and water slowly trying the last resistance path goes into the washer drain pipe and once it reaches specific level the siphon effect happens and water starts coming into washer?
Then after next pumping cycle washer drain pipe is empty and take some time to fill it back to specific level before siphoning again.

I am thinking to redo the last connector and spit it in two to both left and right side of the sink.
This would leave washer on its own. Probably i would also redo washer setup to look exacly like dishwasher.

So access cap in the middle and sink on left and right.

Or do you think this should be redone to match the old Setup?

Attached is old setup.

 

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[bwspot]

i did some testing and removed the washer pipe and when i poured water over right side of the sink it comes out of the washer pipe connector instead of draining via main drain pipe. When i pour water over left side nothing comes out. Seems like that setup is wrong and sink should not be connected like this with washer drain. I found old photos how this was originally setup when it was working and it makes much more sense as sink was separate from washer.

Still from time to time i still see water coming back when i only use left side of the sink.
So i would like to understand how this works.
Is it because drain for the washer is slightly angled down and water slowly trying the last resistance path goes into the washer drain pipe and once it reaches specific level the siphon effect happens and water starts coming into washer?
Then after next pumping cycle washer drain pipe is empty and take some time to fill it back to specific level before siphoning again.

I am thinking to redo the last connector and spit it in two to both left and right side of the sink.
This would leave washer on its own. Probably i would also redo washer setup to look exacly like dishwasher.

So access cap in the middle and sink on left and right.

Or do you think this should be redone to match the old Setup?

Attached is old setup.

Can anyone recommend what should be the correct setup?