I had the same problem as you with the Playstore updating apps automatically. When I already had the setting set to never.
What I did was make sure my setting was turned off. Then I went to the phone settings>apps, scrolled down to the Playstore. Tapped on that, then storage, and cleared cache and data.
Then relaunched the Playstore, selected the account again. Then made sure in Playstore settings the auto update is showing never. If not select it. I've never had a problem afterwards.