For example, if you have selected SIM 2 to send an MMS (Multimedia Messaging Service - which is a standard way to send messages that include multimedia content to and from a mobile phone over a cellular network.), the MMS will be sent / downloaded with the mobile data of SIM 2. It will automatically switch back to the mobile network of SIM 1 when the MMS is sent / downloaded.
This is the design of Android system, not a malfunction of your phone.
* Pictures, settings and paths might differ from those of your phone, but that do not affect the description in this article.
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