I have problem with mobile data while in roaming - it can be disabled completely. Mobile data is stopped directly from icon and "when in roaming - OFF" is choose. I can't surf in Internet via browser or Messengers, but still my phone consume randomly 10-20kb of data from time to time and this cost big expenses in foreign countries. I check with mobile provider in my country Bulgaria and they show me that my phone do more than 20 times connection to the internet for 1-2 seconds per 24 hours during my business trip even at night. For 5 days trip my phone download 400KB, but my provider A1 charge me 50 Euro for this.
Only way to avoid this is to use "Airplane mode" or to delete completely all APNs. Same problems I see on other forums, so seems that problem is not only on my phone.
With Android 8 everything works perfectly, I never had such problem with mobile data consuming when mobile data is stopped when in roaming.
This problem happen after update to Android 9 Pie.
I can't find which process do this when mobile data is disabled, but suspect that this is Google process that work in background and have permissions to enable data connection even when mobile data is OFF (disabled).
My phone is Nokia 8 Sirocco with Android 9 Pie.