I noticed this unexpected behavior of my Galaxy s10e. Currently only SIM2 is present in my phone, and mobile data are switched off on the phone. I found that my mobile operator charged me twice for the use of mobile data (even if the mobile data was switched off on the phone). Looking at the details of the bill of the mobile operator, the time at which the data usage was done by the phone was the time in the morning when I switched off the flight mode. It seems, hence, that when the flight mode is switched off and the phone switches on the radio part, an attempt of using mobile data is done irrespective of whether the mobile data is allowed to be used. My mobile operator told me that the traffic generated was 1284 bytes.
It is 4G/LTE that uses a littlebit data every once in while, even when turning off mobile data switch.
Force your network to 3G/2G under mobile network settings and it will be resolved.
Thanks for your answer. I will keep an eye on it. I kept the phone swithced on for long time, and it seems that there was no data traffic generated. Moreover, when I switched the phone on (from it being off), there was no billing for data usage. It seems that mobile data was used only when the flight mode of the phone was switched off.