ggplot2: add title changes point colors <-> scale_color_manual removes ggtitle

Question

I am facing a silly point color in a dot plot with ggplot 2. I have a whole table of data of which i take relevant rows to make a dot plot. With scale_color_manual my points get colored according to the named palette and factor genotype specified in aes() and when i simply want to add a title specifying the cell line used, the points get colored back to automatic yellow and purple. Adding the title first and setting scale_color_manual as the last layer changes the points colors and removes the title.

What is wrong in there? I don't get it and it is a bit frustrating thanks for your help!

Here's reproducible code to get my whole df and the subset for the plots:

# df of data to plot
exp <- c(rep(284, times = 6), rep(285, times = 12))
geno <- c(rep(rep(c("WT", "KO"), each =3), times = 6))
line <- c(rep(5, times = 6),rep(8, times= 12), rep(5, times =12), rep(8, times = 6))
ttt <- c(rep(c(0, 10, 60), times = 10), rep(c("ZAc60", "Cu60", "Cu200"), times = 2))
rep <- c(rep(1, times = 12), rep(2, times = 6), rep(c(1,2), times = 6), rep(1, times = 6))
rel_expr <- c(0.20688185, 0.21576131, 0.94046028, 0.30327675, 0.22865200,
              0.92941881, 0.13787508, 0.13325281, 0.22114990, 0.95591724,
              1.03239718, 0.83339248, 0.15332420, 0.17558160, 0.22475604,
              1.02356351, 0.77882000, 0.69214403, 0.16874097, 0.15548158,
              0.45207943, 0.28123760, 0.23500083, 0.51588856, 0.1399634,
              0.14610184, 1.06716713, 0.16517801, 0.34736164, 0.64773650,
              0.18334429, 0.05924757, 0.01803593, 0.86685230, 0.39554685,
              0.25764805)

df_all <- data.frame(exp, geno, line, ttt, rep, rel_expr)
names(df_all) <- c("EXP", "Geno", "Line", "TTT", "Rep", "Rel_Expr")
str(df_all)
# make Geno an ordered factor
df_all$Geno <- ordered(df_all$Geno, levels = c("WT", "KO"))

# select set of whole dataset for current plot
df_ions <- df_all[df_all$Line == 8 & !df_all$TTT %in% c(10, 60),]
# add a treatment as factor columns fTTT
df_ions$fTTT <- ordered(df_ions$TTT, levels = c("0", "ZAc60", "Cu60", "Cu200"))
str(df_ions)

# plot rel_exp vs factor treatment, color points by geno 
# with named color palette
library(ggplot2)
col_palette <- c("#000000", "#1356BC")
names(col_palette) <- c("WT", "KO")

plt <- ggplot(df_ions, aes(x = fTTT, y = Rel_Expr, color = Geno)) +
  geom_jitter(width = 0.1)
plt # intermediate_plt_1.png
plt + scale_color_manual(values = col_palette) # intermediate_plt_2.png
plt + ggtitle("mRPTEC8") # final_plot.png

images: