This isn't a bug. Any full grown tree has a small chance to turn into a mushroom tree during fall.
The relevant code is in
StardewValley.Farm::DayUpdate
C#:
if (terrainFeatures.Count() > 0 && Game1.currentSeason.Equals("fall") && Game1.dayOfMonth > 1 && Game1.random.NextDouble() < 0.05)
{
for (int tries = 0; tries < 10; tries++)
{
TerrainFeature t = terrainFeatures.Pairs.ElementAt(Game1.random.Next(terrainFeatures.Count())).Value;
if (t is Tree && (int)(t as Tree).growthStage >= 5 && !(t as Tree).tapped)
{
(t as Tree).treeType.Value = 7;
(t as Tree).loadSprite();
break;
}
}
}
Starting Fall 2, any
untapped common tree has a 5% chance of turning into a Mushroom Tree. If you're unsure of the details, you can always check the wiki.
Thats mean my sweet apple tree would turn into a big mushroom? Oh my.
Fruit Trees are not "Tree" in the code but rather "FruitTree", so they will never turn into Mushroom Trees (that wouldn't be a very nice thing for the game to do).
trees turning into mushroom trees is normal since starting fall 2nd the game checks tiles and sometimes turns a fully grown tree into a mushroom tree. However I am not sure if it's supposed to do that on Trees that have a tapper on them - in the compiled code it looks like it's supposed to ignore tapped trees (&& !(t as Tree).tapped) but I could be wrong.
Either a Heavy Tapper isn't properly registering as a tapper, or the Mahogany Tree isn't properly registering as a "tree", or something else entirely, that the developers can figure out better than I can.
Cheers!