I have never used those options in twrp and know nothing about them i always flash normal way, do you mean flash gapps by mounting it and using the adb sideload in twrp?Interesting... if it does that, maybe try going into `Advanced` and choose `Mount/Unmount System` first and then try the adb sideload again, see if that makes it happy.
You can also enable ADB from the Advanced menu, and `adb pull /tmp/recovery.log` to get the recovery log off the phone to save here so we can take a look at it more.
(BTW there is nothing in the advanced tab in twrp about Mount/Unmount System??)
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