Maybe it should, but in my experience Android is perfectly happy to automatically update Play Services and Service for Instant Apps (whatever the hell that is)

Maybe it should, but in my experience Android is perfectly happy to automatically update Play Services and Service for Instant Apps (whatever the hell that is) over the cell network, despite settings being set to manual update on WiFi. Which is a Windows 10 level of obnoxiousness. So I wouldn't be surprised if they were sending stuff over customer's metered connection.