Hi Qi,
Thanks for the input. This listbox is an example (and I have several of them), but they aren't the only "custom input fields" I have - some are text or a check box as well. Basically, the "default" form controls on a SharePoint list don't fill my needs, so I've added more friendly ones to the data cards as an input field.
So to answer the question about in the form or outside of the form - I've placed the majority of them within data cards that I've added to the form using the "add fields" tool. So the workflow for a "native" dropdown, for example, is:
- Add the item (list column) through the "add fields" tool
- Add an input control to the resulting data card (i.e. if it's a dropdown, add a list box)
- Set the default value of the default dropdown to the list box selected value
The issue here is that once the form is saved, the list box selected value disappears (I have the default controls hidden for UI purposes)
For text box arrays, like this:

I have a combined "medical plan information" data card and add these input controls to the data card to simplify user entry. You'll note that there are three different input types here: text, dropdowns, and check boxes. In this case, I have a concatenate formula to combine all of the data points into a single field for our reporting purposes, but I'm still having the same "bi-directional" issue where the data that's been input disappears upon save.
FINALLY, I have check boxes and text boxes that sit outside of the form container. The check boxes show and hide sections in the form, and I think I can get around that piece with just a separate "data list" to hold that information (right? Am I making sense?). However, the text boxes that sit outside of the form DO tie to the form and I have the same issue with the "bi-directional" data - I can get the data TO the SharePoint list, but it disappears from the input.
I know this is kind of long-winded and a bit convoluted. I super appreciate any assistance or ingenious ideas!