One of the network managers is apparently set to enable DHCP on the interface it manages, that’s the only reason that I can think of why a device set with a static IP is switching to using DHCP.
You’re going to want to check the systemd units (sudo systemctl list-unit-files --state=enabled, if that doesn’t work you can replace the state flag with –no_pager and grep for enabled ) to see what is managing the interface(s) and then ensure that their config is set to static. You may have two conflicting services like systemd-networkd and NetworkManager fighting.