There’s a couple threads on it, see link in next paragraph. The glass avionics screens, at least the Garmins, are rendered in a way that doesn’t expose any geometry or motion information to the DLSS upscaling tech, which creates two problems:
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Static blurriness: the graphics and text are rendered at a lower resolution and don’t scale up as well as physical geometry, like raised labels and dials on an analog gauge. (They do, however, scale up better than TAA + the same amount of upscaling, IMO, but look much worse than native resolution.)
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“Ghosting” / dynamic blurriness: as things move around on the little screen, especially numbers changing or moving, they become harder to read because DLSS uses past frame data in its scaling algorithm; this leaves a “trail” on moving items because, unlike 3d geometry, none of that motion or boundary information is available to the scale-up algorithm. This can significantly affect legibility of the altitude and airspeed tapes, and vertical speed indicator, on the Garmins.