In emerging optoelectronic systems designed for AI and neuromorphic integration, group-IV photonics and particularly (Si)GeSn-based lasers are of significant importance due to their CMOS compatibility and potential for scalable, on-chip optical interconnects. Towards further improvement of material and device, here we investigate self-heating in a state-of-the art continuous-wave (Si)GeSn microdisk laser using stroboscopic full-field X-ray diffraction microscopy with 10 ns temporal and 150 nm spatial resolution. This study quantifies and visualizes heating induced by the electrical pumping pulses, revealing contact-dominated Joule heating as the primary cause of dynamic temperature changes in the device. Moreover, we confirm that heat localized at the contact region does not effectively propagate into the active lasing layer due to the low thermal conductivity and high heat capacitance of the SiGeSn/GeSn multi quantum wells. These finding are valuable in guiding future thermal design of group-IV photonic devices.