Its not that they cant be used in conjunction with each other (of course they can), its just that with your current wiring, there is a ground current circulating somewhere and so just like a rising tide lifts all boats in the harbor, a common ground current raises all voltages on the module.
Lets take a look at the wiring diagram for each module…
Starting with the GRV-IV-24
Notice how all the first bank of 12 inputs share the same ground.
Same with the second bank of 12 inputs.
Also notice how the two banks are isolated from each other… we will get back to that in a sec…
Now lets look at the GRV-IVI-24
Notice how each input has nothing at all in common with any other input.
This module is fully isolated.
No matter what one input ground reference is, it can in no way impact any other input.
This is not a bad summery of the situation.
So a few tips.
If you are dealing with mv inputs, things are going to be more sensitive.
The power supply should be tested / inspected.
The temperature sensors used should be understood, if they are metal tipped, are the tips grounded, can they be safely grounded?
Is there more than one ground point in the setup?
If the sensors are different types and only one type is metal, then move it to the second bank on the module so it does not impact the other sensors.
Hope that helps you get up and running.