mobile data toggle keeps turning off - iOS12.3.1

mobile data switch turns back off for most apps (except safari, apple support, Instagram) after turning it on and when leaving general settings and opening back again

First when I open mobile data settings for apps it takes some seconds and the wheel turns:

when I turn the toogle on for an app it first stays on and it looked like this:

but when I completely leave the general settings menu and open it again go to the apps mobile data settings it’s turned off again:

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What I have done so far:

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iPhone on/off

networks settings reset

Airplane mode on/off

Mobile data on/off

hard reset

mobile carrier update

used restrictions menu

What I have NOT done (and don’t want yet as I am on a vacation in Sweden and don’t have wifi):

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reload from backup

set up as a new iPhone

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The bug even turned the switch back off when I used “restrictions” for mobile data settings as it was recommended in another posting which seems to me as a big security breach !!!

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ONE additional observation:

one time the toogle for mobile wifi support and icloud drive mobile support (pic below in german) actually appeared completely duplicated under each other and could be turned on and off. Unfortunately this appeared only one time and I could not take a screenshot, but it looked like this:

Any more recommendations ?