Getting choices for a combobox (drop down) from another SharePoint list

Question

I'm using PowerApps to customize a SharePoint form and am running into an issue. I have a combobox (dropdown) for which I want to pull the choices from another SharePoint list (we'll call it the "Source SP"). To accomplish this I have created a Collection at the "App" level of the Tree view under "OnStart": Collect(colSPListItem, 'Source SP'.Title).

Then under the "Items" property of the drop down field in question I'm using colSPListItem, for "DisplaFields" and "SearchFields" I'm using ["Items"]. This works well, when I start a new SharePoint form I am able to see the drop down choices pulling from the "Title" column of the Source SP list. The issue is that when I submit the form, the drop down field does not retain the selection. I am new at PowerApps and I've tried spending a considerable amount of time at no success.