I will release here my updated "boot.img" files for the Xperia XA Ultra Dual Sim.
You must have an unlocked bootloader to flash with Fastboot.
You don't need to be on the same firmware as the boot image that you want to flash.
You have to flash SuperSu again from TWRP just after flashed new boot to avoid bootloop.
All boot images have "ro.secure=0" and "ro.debuggable=1" in default.prop file
and "forcencrypt" of internal memory switched to "encryptable" in fstab.mt6755 file.
The Fastboot command is:
fastboot flash boot NAMEOFIMAGE.img
12/28/2016: From 36.0.B.2.109 kernel sources and Ramdisk from 36.0.B.2.109 firmware.
04/01/2017: From 36.0.B.2.146 kernel sources and Ramdisk from 36.0.B.2.146 firmware.