If you use a Fabric Attach managent vlan to do ZTP+ on an unconfigured switch, the ZTP+ process will start normally, but in the end the default vlan will be used to configure the management IP address on the switch. When that is done ZTP+ looses the connection to XMC and the switch will reboot.
To overcome this you need to create a vlan to the site on XMC with the tag of the FA management vlan. (do not use SYS_VLAN_<tag> but use another name!)
You need to add this vlan to the interswitch port template as a tagged vlan.
During the ZTP+ process it will create this vlan on the switch and assign it the management IP address and ZTP+ should complete.