Sorry for the delayed answer.
@Gertjan That diagram is just to represent how it was before I change Time Capsule from that Site A to Site B, now both devices are in 10.0.10.0/24. The idea that I tried to pass it was to show you that it was indeed two differente physical NICs per device.
Both Firewalls, have DHCP servers in place, but 10.0.0.0/24 uses WS DHCP server, and all of those are working properly for months/years.
In 10.0.10.0/24, there is at least one device with a local setup IP (10.0.10.6). But not signed for those two IPs, and I have already changed from static to dynamic IPs in DHCP server but got the same result.
Its like both devices have been assigned with the same IP, but it was not, so I really dont know why its happeaning.
After 3 days being massively spammed by arpwatch, it stops, Time Capsule using the right IP.
Since I am not physically present in that site, I can only assume that someone have turned off that Apple TV.
@stephenw10 I have made that several times. I didnt made any sniff attempt, but next time I will have to do it, because it wasnt normal.
If I got this issue again, I will let you know, even if I find the reason for this to happean.
Thank you all for the help, always appreciated!