Cannot save multi user Picker field with Inactive user

Steps to reproduce:

1. Create (or choose) a custom "User Picker (multiple users)" field.
2. Assign an active user to it
3. Make the user inactive.
4. Make sure that the inactive user is still present among field values
5. Try to add another (active) user to the field

Expected result:

• New (active) user added
• Old (inactive) user still present.

Actual result:

• Error: User '<old user name>' is not valid for this user picker and it won't allow you to save the changes.