There doesn't appear to be any crimson/warped fungi in the Nether, I can confirm that this is a bug because I tested it in 2 different launchers (MultiMC and Official Minecraft Launcher) and in 3 different worlds, a single-biome world (creative, no seed) and 2 normal worlds (creative, no seed).
Environment
Windows 10 64 bits, Java 8 1.8.0_271-b09, NVidia GeForce GTX 1060 3GB, Intel Core i5 8400 CPU @ 2.80 GHz 2.81 GHz, 6 cores, 16 GB RAM (never had problems before, I use 15GB for Minecraft), Using Dual Boot with PrimeOS. My Minecraft is 100% clear, unmodded.
Duplicate of MC-203574