Mushroom blocks use their default map colors even if the upwards face is not present
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If the top face of a mushroom block is not present, it will still use the color that the mushroom block type typically uses. They should probably use the "Sand" or "Terracotta_White" color. Relates to MC-139453 and MC-175228.
Intended mechanic of maps, for example, a horizontal Acacia Log won't be gray on a map.