Mooshrooms do not respawn in mushroom island biomes

Started a new map centered in a mushroom island recently. As part of my build, I built a pen for all the Mooshrooms I could find on the island. (about eight) Unfortunately, they all got wiped out during a lightning storm (presumably by two strikes since there were two skeleton horse traps in the pen.) I have searched the entire island repeatedly looking for any respawns, but after several hours, I couldn't find any. I can only conclude that once the initial spawns are gone, they are gone for good.

Steps to reproduce

• Make a creative world using mooshroom biome seed 1404986100
• Make sure cheats are on.
• Fly all around the biome for a while until enough mooshroom cows spawn.
• Now type /kill @e
• Observe that the mooshroom cows don't ever re-spawn.