Hello,
In CSCO world the same scenario results in ARP reply dropped by Cisco with "Wrong cable" syslog message
http://www.velocityreviews.com/threads/wrong-cable-interface-gigabitethernet0-1.379622/
if You are acquainted with the guys using Cisco with same upstream out there You may want to ask some pointed questions.
OTOH, You may get away with this upstream behaviour using SRX if You migrate from RETH to 2 separate outside physical interfaces with restricted proxy-arp enabled on each.
HTH
Thx
Alex
P.S. Removed extra space at the end of URL