I believe the SharePoint List Lookup control is not yet supported on Nintex Mobile.
Where could I find an ETA of forthcoming features for the Mobile platform?
Could someone recommend a way to display a filtered content in a drop down selection based on a previous value?
The list lookup control was recently added to the Nintex Mobile for O365 product. Make sure that your mobile app is up to date and your forms for O365 isnt pending an update. Let me know if you're still seeing this issue.