Strange `a` on the legend of a `ggplot2` graph

Question

I am doing a linear interpolation using ggplot2, with two series which generates a legend, reporting Primer A and Primer B.

On the legend appears a strange a on the left of the symbol which I am not able to remove.

I saw another similar topic: "Remove 'a' from legend when using aesthetics and geom_text" but it is not the same case, in my case I am using geom_text_repel() in that one somply geom_text().

What could be the problem?

library(ggplot2)
library(ggrepel)

# Dataframe creation
RawData <- data.frame(
  Time_h = c(0, 10, 20, 30, 40, 50, 60, 0, 10, 20, 30, 40, 50, 60),
  Percentage_Curing = c(100, 95.48, 91.35, 87.9, 84.77, 81.97, 79.37, 100, 92.15, 84.29, 76.65, 69.6, 63.42, 59.66),
  Ref = c(
    "Primer A", "Primer A", "Primer A", "Primer A", "Primer A", "Primer A", 
    "Primer A", "Primer B", "Primer B", "Primer B", "Primer B", "Primer B", 
    "Primer B", "Primer B"
  )
)


print(data)



is.num <- sapply(RawData, is.numeric)           ##Reduces decimal figures
RawData[is.num] <- lapply(RawData[is.num], round, 2)


#Force first point: (0;0)
RawData$weight  = ifelse(RawData$`Time_h`==0, 100, 1)


library(tidyverse)
library(investr)


#Creation of the linear model
(rawdgrps <- RawData %>% group_by(Ref) %>% group_split())

models <- map(rawdgrps,
              ~nls(formula=`Percentage_Curing` ~ (a_ * `Time_h`) + b_,   
              start = list(a_ = 10, b_ = 5),
              weights = weight,
              data = .x)
              )

(predictions <- map2_dfr(models,rawdgrps,
  ~bind_cols(.y, investr::predFit(.x, newdata = .y, 
                  interval = "confidence", level = .8) 
)))





### GRAPH CREATION



Graph <- ggplot(data=predictions, aes(x=`Time_h`, y=`Percentage_Curing`, col=Ref)) + 
  geom_point( shape = 1, size = 2.5) + 
    geom_line(aes(y=fit)) + 
    geom_ribbon(aes(ymin=lwr,ymax=upr),alpha=.1,linetype=0) +
  scale_color_manual(values=c('#f92410','#644196')) +
labs(color='') + 
theme(legend.justification = "top") +
geom_text_repel(max.overlaps = Inf, 
segment.curvature = 0.1, 
segment.ncp = 3, 
segment.angle = 60, 
force = 1,  
size = 2.5, 
angle = 0, 
segment.alpha = 0.25, 
segment.size = 0.25,  
nudge_y = 0.0, 
egment.square =FALSE, 
min.segment.length = 0, 
family = "Calibri", 
box.padding = 1.5, 
aes(label=paste0( "Weight ", `Percentage_Curing`, " %", "\n", "Time: ",`Time_h`, " min")),
    size = 3,
     hjust=0.1)
Graph