Are you setting it to a static IP on the same VLAN it's using the DHCP IP from? Or, changing to an entirely different subnet/VLAN? Typically this happens when the address you're attempting to use is failing to work and the AP (or even a switch or MX) will defer back to using DHCP.
I think I found your network. If this is yours I see you have the AP static IP set and VLAN 13 specified. You also have native VLAN 13 on the upstream port. You can't tag the AP mgmt IP and have it native/untagged on the switch. Easiest fix just delete the VLAN tag on the AP static IP config.
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