Giant mushrooms (red or brown) no longer appear to spawn for me naturally in 0.13.0 in iOS. They no longer appear in the roofed forest biome where I've found them before. I am also no longer able to plant them anymore either where I've been able to in the past. I checked on 0.13.0 on Windows 10 Edition and everything appears to work there as expected (same seed, they spawn on Win10 but not iOS), so maybe it's an iOS thing only (not sure, don't have Android).