Changing /system/build.prop issues on fully rooted and DM-verity removed device

SO, I have an MT6739 dveice with Android 8.1, it's not A/B seamless, and it is not TREBLE. I have used a tried and tested method to remove DM-Verity. On this device it was in the DTB, in the boot and also in the vendor partitions. I use Magisk to get me root and I have removed and installed lots of /system/apps, changed the audio, bootanimations etc on the device. I even changed the backgrounds and tweaked the Launcher. However whenever I come to change anything in /system/build.prop the device bootloops! I have tried this with Magisk DM-verity ON/OFF with no effect. I also copied back a build.prop from my PC and it worked fine again. So, something must be looking up the build.prop and checking for changes. All I have done is change the ro.product.model property to be super sure nothing in the build.prop is incorrect. I have tried with setting permissions (644, for some reason they are 600 (RW - - ) on the device so tried that too). There is a build.prop in the /vendor partition. i have heard of this checking the fingerprint of the system/build.prop, but they are different anyway on the stock ROM. I also tried just copying the build.prop from /system into here so they were identical. Fresh out of ideas!! Does anyone know what is going on here. I can literally change anything I want on the device as normal/pre-verity days except for the build.prop settings.