This article explains an issue where the DHCP client like a switch sitting behind SRX is not assigned a statically reserved IP.

DHCP client like the switch lying behind SRX might get any IP from a DHCP pool instead of statically reserved IP configured in MIST wan edge template.

This issue can be seen if the chassis MAC address of the DHCP client is incorrectly configured in the static reservation table under DHCP configuration on MIST portal. This could happen if the old client machine which was assigned a statically reserved DHCP IP gets replaced for any reason and you miss to update the MAC address in the static reservation table from the MIST UI. You need to replace the old MAC address with the new MAC address for the new client to get a reserved IP address.