Hi,
I see this is quite an old post but I am currently having the same issue and the solution given here is not working for me. I would really appreciate if someone could help me 🙂
Here is the dropdown, connected to the SharePoint table from which I select the Legal Entity field:

And this is the piece of code in my Form's DataSource:
LookUp('DEI', 'Legal Entity' = leDropDown.Selected.'Legal Entity')
For some reason, when I select something from the dropdown, nothing appears in the Form.
Is there anything I am missing?
Thank you in advance!