"decompose" method doesn't work well in combination with differencing

[ellisp]

for example, here are the four different seasonal adjustment methods with differencing on:

model5 <- xgbar(AirPassengers, maxlag = 24, trend_method = "differencing", seas_method = "dummies")
model6 <- xgbar(AirPassengers, maxlag = 24, trend_method = "differencing", seas_method = "decompose")
model7 <- xgbar(AirPassengers, maxlag = 24, trend_method = "differencing", seas_method = "fourier")
model8 <- xgbar(AirPassengers, maxlag = 24, trend_method = "differencing", seas_method = "none")

fc5 <- forecast(model5, h = 24)
fc6 <- forecast(model6, h = 24)
fc7 <- forecast(model7, h = 24)
fc8 <- forecast(model8, h = 24)

par(mfrow = c(2, 2), bty = "l")
plot(fc5, main = "dummies"); grid()
plot(fc6, main = "decompose"); grid()
plot(fc7, main = "fourier"); grid()
plot(fc8, main = "none"); grid()