Multiplexer giving same values for different channels when soldered

Im using a Multiplexer to read multiple Potentiometer to do things with it. Ive come far enough with prototyping boards and made sure all connections are working and I get the data I need. With these prototyping boards, where I put them in without soldering, it works flawlessly.
Now I need to solder them and here is my problem: I soldered these Multiplexer 6 times, each with a unique Multiplexer and unique solding Boards and cables, so nothing was reused to make sure i dont use components that got broken. All cables, each time before soldering, were tested. No unwanted bridges were soldered. But every time I was done, one potentiometer changes values of channels, where its not connected to. This means, with my 8 Potentiometer, 1 Potentiometer changes like 5 values. However, logically, one potentiomer should only change the value of its own channel.

So i thought maybe its because the Multiplexers get destroyed somehow each time when I solder them. Okay, so ive bought this and soldered that and then later put the Multiplexer in. Same result. I have no clue why it does that and im frustrated. Again: my program works when its on prototyping boards and values are correct, but not when I solder them. Where could be the problem? And as said, connections are good soldered and there are only connections where i want them to be.

Does the tip of your soldering iron, when it is plugged in and heating, show any voltage between it and your mains power ground? AC volts?
Since you are dealing with CMOS, are you careful to avoid static electricity when you are working with the device?

Yes it does as it seems. Does this mean I could have manipulated parts with it while soldering?

It means you could have electrocuted some components. Get a descent soldering iron.

There could be an error in your program. Perhaps if you show the sketch you are using, someone might spot an error.

It sounds like the boards do not match the prototype. Just a SWAG but it sounds like the pot ground is floating and the wiper that controls all of them feeds via the open ground. I would look for that first as the boards are not known good.

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