With save analysis JPEGs enabled, zmc logs:

WAR [zmc_m5] [Target image is already colourised, colours: 1]

Cause: since decoder-native passthrough, packet->image is typically YUV420P/YUVJ420P regardless of the monitor's configured Colours. The zone alarm highlight is still built in the configured format (RGB24/RGB32), so Image::Overlay() dispatches the RGB-highlight-onto-YUV420-target combination into the 'target is 1 byte per pixel' branches, which call Colourise(). Colourise() refuses on non-GRAY8 targets (the logged warning) and returns, after which the overlay loop walks the planar YUV buffer as packed RGB, corrupting the saved analysis frames.

Fix: in Image::Overlay(), gate the Colourise() branches to true GRAY8 targets and add YUV420-target branches that convert only the marked (non-black) RGB highlight pixels to YUV (luma + shared chroma), honouring the highlight's subpixel order.