Sony's My Xperia Theft Protection breaking certain bootloader unlocked models
MXTP is designed to prevent the bootloader on an Xperia device from getting unlocked. If My Xperia Theft Protection is enabled, the consumer will not be given the opportunity to unlock the bootloader, even if the phone or tablet comes out of the box with that option.
A major problem is taking place with Xperia models that already have the bootloader unlocked when MXTP is enabled. Since the unlocking has already taken place, the security feature is trying to shut the barn door after the horse has bolted. And in the process of trying to prevent something that has already occurred, the phone becomes damaged permanently.
Sony has changed some of the wording on its Unlock Bootloader site regarding the use of MXTP. Under the section titled Risks, Sony reminds device owners not to enable MXTP after unlocking their device. The manufacturer also reminds users to turn off MXTP before unlocking the bootloader on their phone or tablet.
Right now, there are no custom ROMs available for the Sony Xperia Z3+, Sony Xperia Z4 Tablet, Sony Xperia C4 and the Sony Xperia M4 Aqua, which eliminates the need for consumers to unlock the bootloaders on these models. Additionally, Sony doesn't allow consumers to unlock the bootloaders on these devices for the time being, anyway. That might change though, once the MXTP is fixed.
source: Sony, XperiaBlog via AndroidCentral