Your MBPro boots into Safe boot mode when the shift key is pressed.
Karabiner does not affect the keyboard until the system loads that software, so the shift key is
still stuck, and you boot into Safe boot mode.
I did some boot testing holding shift and/or option. Shift boots to Safe Boot, of course.
Option key only, boots to the boot-select screen.
Holding both, my Mac boots normally - no safe boot, no boot select screen. Just a normal boot.
So, bottom line, your shift key is still stuck. Karabiner doesn't fix that, it only disables the shift key AFTER the software loads, so you have a dead shift key while you are using your MBPro.
It is still stuck, however. And, your work-around is holding the Option key during boot. Again, I think holding Option during boot makes your Mac ignore the stuck shift key.
And, when you finish booting, the Karabiner software comes into play, disabling that shift key.
(You need to replace the keyboard/top case
)