Hi Ale,
There is a known issue with Gratuitous ARP handling in GW HA mode in code below 3.4.0012 and below, we should release a code with the fix soon (Until End Of March),
for now I would suggest clearing the Arp Entry from all of the GW's with the below commands:
Entire arp table for the Proxy-arp interface:
switch# clear ip arp interface proxy-arp X
when X is the proxy-arp Interface id
or a specific entry:
switch# clear ip arp 1.1.1.133