This polygon shape does not lay out content cleanly.
The line breaks are odd and first fit seems to have pushed down content in the center.
Created attachment 185045 [details]
screenshot of bug
Created attachment 185046 [details]
Created attachment 188221 [details]
Screenshot with reflex vertex fix and corrected coordinates.
The specific problem you'd pointed out in the bug report no longer occurs with the reflex vertex fix (https://bugs.webkit.org/show_bug.cgi?id=107568) however the first line of text from your example still doesn't fit. This isn't because of a problem with webkit, it's because the top and bottoms of the figure aren't really horizontal. A straightened verison of the shape looks like this:
-webkit-shape-inside: polygon(0% 18%, 14.726% 11.66%, 12.222% 4.588%, 0% 0%, 75.732% 0%, 63.411% 4.697%, 61.07% 11.612%, 75.732% 18%);
Fixing the coordinates here and the corresponding SVG polygon coordinates produces the expected layout. See the attached screenshot.
*** This bug has been marked as a duplicate of bug 107568 ***