I have explored this problem further and found that it is not an upgrade issue. The problem can be reproduced simply by creating an entity in v3.0, creating a simple data grid and associated data maintenance form. Then create a data set to query the data along with a simple report to show the data on screen.
Once you have created all of the above add them to a form with two tabs, show the data set simple report on one tab and add the data grid to the other tab Run this and the system will fail to show any data on both tabs!
I had the same problem. You are using the report widget without using a report pane.
Try this: add a form somewhere in your project. Add the report pane widget to it. Restart and that solved the problem in my case.
It seems that Mendix forget to deploy the report JS files when you do not use a report pane but use the report widget without a report pane. I hope that this work-around helps for you, but it's still a bug.
Appears your project was not converted properly. I suggest filing a ticket with the old project and the converted project. Which modeler versions did you use to convert from 2.4 to 3.2.1? I don't think it's possible to do a straight up conversion right?
Also, did you make sure you had no (deprecation) warnings left during every conversion step?