Hi,
On-board LED controller on Razer Death Adder Elite V1 (RZ01-02010100-R3G1) has likely some conflict with one of on-board LED controllers (eVision VS12L03A I2C chip) on GMMK v1 keyboard when both devices are handled by the same USB controller.
It manifests with half of LEDs going off on GMMK keyboard, when Razer mouse is plugged-in. When devices are plugged-in in a way that different USB controllers are handling them, there is no such issue.
Similar issue albeit with different devices, was reported here:
Please note that issue is orthogonal to any LED software present or absent.
It is sufficient to plug-in those devices to computer (and same USB controller on it) and even sufficient to have them initialised by UEFI, no need to even start OS. Then, when toggling some RGB modes on keyboard via shortcut, eventually causes half of LEDs go off when Razer mouse is also plugged in.
Issue happens regardless what firmware is installed on GMMK keyboard - stock or QMK, so this appears to be either some heavy EM interference or indeed LED controllers colliding on their interface while being present on the same USB bus?
What LED controller chip is installed in Razer Death Adder Elite? Could it be that also VS12L03A?