You can not have a popup inside of a repeating group…popups can only live on a page or in a reusable element, so despite having placed the popup into a reusable element, which can be done, you then put the reusable element into a repeating group and expecting the popup to show within the cell of the repeating group, which can not be done.
If I understand correctly, you might be experiencing the issue described on this page of the documentationn, which states in part…
Generally speaking, it’s not good practice to have a popup inside each cell of a repeating group. For one thing, it needlessly adds “weight”to the page.
There are at least a couple alternative approaches where the popup resides outside the repeating group. One would be to store a reference to the “selected” cell’s thing in a custom state, and then reference that custom state from the popup.
Just to be clear though (for others who might stumble upon this thread), it is perfectly valid to have a Popup inside a reusable that’s inside a RG (just as you can have a Group Focus within a reusable inside a RG).
I’ve worked on a number of projects which use that design pattern (from reputable agencies). It’s not an uncommon practice. It’s just not a particularly good one IMHO.
The “disappearing data” issue you were experiencing is most likely due to the list being re-sorted after a new item is added, thereby resulting in the Popup referencing a different cell, so its data source changes (and thus the display values disappear).
Anyway, I think you’re on the right track by moving the Popup outside the RG.
Good luck!
EDIT
The issue you’re experiencing could likely be fixed while keeping the Popup where it is, but refactoring so that it’s outside the RG is a better approach IMO.