Steps to reproduce:
A-Add some fields to the User entity
1-Add the 'First Name' field (brut text)
2-Add the 'Last Name' field (brut text)
3-Add the 'Company' field (Entity Reference to Company content type)
B-Create the view
1-Create a user's view of type "Entity Reference"
2-Add some fields: name, first name, last name
3-To make sure it will works, in the format settings dialog, click on all fields for the search fields option
4-Add a contextual filter based on the Company field: Default Value > "Content ID from the URL"
C-Add a User's ER field to the Company content type
1-In the manage field page, add a new User's Entity Reference field [ILLIMITED VALUES]
2-In the "Reference Type" Setting choose the View>Display we just created
3-In the display form page, choose "Auto-completion"
D-Encode some Users's having the company "MyCompany" as a value of their company's field
Result
1-Go to the edit page of "MyCompany"
2-Try to add a User by typing a letter=> Autocompletion will not work
BUT, if you change the autocompletion widget into a checkbox widget, it will works as only the users of this company will be listed as check box items.
This is clearly an issue with the autocompletion widget