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0.56 s exactly the quantity corresponding to extra-offset to 0.1. But if you produce a SVG file, and open it with a text editor, you will see three pairs of coordinates:
(first bracket)
<text transform="translate(35.1781, 4.6976)" ....
(second bracket)
<text transform="translate(35.1781, 4.6976)" ....
(third bracket)
<text transform="translate(48.2297, 4.5976)"...
As you can see, only the third bracket (corresponding to the extra-offset property) is correctly shifted by 0.1
I also checked that the same issue happens with DynamicLineSpanner. There's a value aroud 0.46, but not exactly 0.46 as the default offset. And it seems to be the same for both OttavaBracket and DynamicLineSpanner....
Where did you pick the 0.46 value? I did grep on the whole tree src, and even if it's set as default_outside_staff_padding_ (axis_group_interface.cc) it must have overridden by something else...
Best,
Paolo