Try to find the form in the dbo.NM_Form table and change the IsDeleted column from 0 to 1.
Follow the steps below to navigate to the Forms table and edit the IsDeleted column.
1. Open SQL Server Management Studio >> NintexForms_DB >> dbo.NM_Form
2. Query the table to display the records
3. Locate the form in the table with the help of the column "Name"
4. Check for IsDeleted column for the related form and change into 1.
After following the above solution, if the form still appears in the Nintex Mobile app then delete the whole row in the dbo.NM_Form table for the related form.
Note: Nintex is not advised when a SharePoint asset is deleted so to avoid this issue from occurring in the first place the Nintex Form should be deleted first before the content type or list etc.