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Hi @Januskopf, welcome to our community!
I'm sorry to hear about that, must be very irritating.
Is this something that has been occurring since you started using the phone for the first time or did it start to occur after you updated your phone or installed a new application?
When you put the phone in do not disturb mode, are you choosing a specific time when it should end, or are you using "No time limit"?
As @uliwooly recommended, safe mode is a great tool to use to make sure that a third party application isn't the cause of the do not disturb mode is being switched off by itself.
I would also recommend that you go in to Settings > Sound > Do not disturb > Automatic rules and make sure that you don't have an active rule in here that is turning it back on.
Let us know how it goes!