Strange behavior: same IP for different MAC

Hello everyone!

Router: Cisco E900
Firmware: Tomato Shibby v140
Subnet: 10.4.1.0/26
DHCP pool: 10.4.1.31-50

Never an issue, until now, when a POE switch (don't know brand/model) was added for 2 IP cams.
If 1 single cam is connected, then no problem. When 2nd cam is connected then the issue.
If both cams are connected in reverse order, same issue.

Any thoughts about this?
POE switch with bad MAC table?

Thanks in advance!