This is caused by the inverse-order polgyons after mirroring. Adding
a union_ before offset_ex fixes this bug.
Github: #1621
Jira: 2738
Change-Id: I885a3ca6af4e5fadb2158d913cf67525bfffa91e
(cherry picked from commit 4ea95d89e7ec058ac21218e1839cfe16b486893a)