Asymmetric midpoint in scale_fill_gradient2 results with trimmed color on the shorter edge

Question

Here is starting example I'm trying to modify:

library(reshape2)

data <- mtcars[, c(1,3,4,5,6,7)]
cormat <- round(cor(data),2)
melted_cormat <- melt(cormat, na.rm = TRUE)

ggplot(
  data = melted_cormat,
  aes(Var1, Var2, fill=value)
) +
  geom_tile() +
  geom_text(
    aes(Var2, Var1, label = value)
  ) +
  scale_fill_gradient2(
    low = 'red',
    high = 'blue',
    mid = 'white', 
    midpoint = 0, # <-- look at this
    limit = c(-1, 1)
  ) 

This code creates a plot:

But when I change the only midpoint to midpoint = -0.5 plot looks like:

In my opinion, the output is not correct since I clearly stated low = 'red' and in the plot the lowest value color is between red and white.

I'm looking for a solution how to maintain midpoint = -0.5 and a full gradient from -1 (red) to -0.5 (white).