Do you have admin control over the ubuntu router?
If yes, you might want to check the shared_media [#] setting 
on that router for the interfaces with overlapping subnets.
(it is on by default, I would try turning it off).

AFAICT, the code does the right thing per rfc1812 when setting
IPSKB_DOREDIRECT if shared_media is turned off.

--Sowmini

[#] https://www.frozentux.net/ipsysctl-tutorial/chunkyhtml/theconfvariables.html