The most likely cause is the phone’s full-resolution processing mode, not the CPU simply “failing to finish.” On this sensor, 200MP capture is a special mode; the camera is designed to work primarily in binned modes such as 12.5MP (and sometimes 50MP), where multiple small pixels are combined for better image quality.

A visible grid in 200MP shots is consistent with artifacts from that full-resolution pipeline—such as de-binning, multi-frame/composite processing, or sharpening/detail enhancement—especially when the phone is hot and you shoot several frames in quick succession. Heat and rapid burst shooting can make processing artifacts more likely, but the pattern itself points more to how the image is being assembled than to a true sensor defect.

So yes: this is plausibly a limitation/bug of the 200MP mode rather than evidence that the sensor isn’t 200MP. Practical takeaway: use the default 12.5MP mode for the best reliability and low-light quality, and reserve 200MP for bright light and slower shooting. If it happens often even after restarting/updating the phone, compare with another device or contact Xiaomi support to rule out a hardware issue.