Vega and subsequent GPUs implement restrictive code signing practices on firmware
AMD GPUs in the Vega generation and later will not load firmware not signed by AMD.
When questioned, AMD claimed that this was in order to comply with Microsoft Secure Boot requirements. See Windows 10.
There are claims that the firmware signing checks only apply when these GPUs attempt to load firmware from EEPROM, not when loading firmware over PCIe at the direction of the OS. As such, it may still be possible to load modified firmware when running Linux.