image_read() dropping frames

Hello, trying to run this simple tutorial on the gganimate page for combining multiple gifs. The repex they provided (shown below) fails with an Error: subscript out of bounds error in the final for loop which is sequentially combining individual frames to create the final gif.

library(dplyr)
library(ggplot2)
library(magick)
library(gganimate)

A<-rnorm(100,50,10)
B<-rnorm(100,50,10)
DV <- c(A,B)
IV <- rep(c("A","B"),each=100)
sims <- rep(rep(1:10,each=10),2)
df<-data.frame(sims,IV,DV)

means_df <- df %>%
               group_by(sims,IV) %>%
               summarize(means=mean(DV),
                         sem = sd(DV)/sqrt(length(DV)))

stats_df <- df %>%
              group_by(sims) %>%
              summarize(ts = t.test(DV~IV,var.equal=TRUE)$statistic)

a <- ggplot(means_df, aes(x = IV,y = means, fill = IV)) +
  geom_bar(stat = "identity") +
  geom_point(aes(x = IV, y = DV), data = df, alpha = .25) +
  geom_errorbar(aes(ymin = means - sem, ymax = means + sem), width = .2) +
  theme_classic() +
  transition_states(
    states = sims,
    transition_length = 2,
    state_length = 1
  ) + 
  enter_fade() + 
  exit_shrink() +
  ease_aes('sine-in-out')

a_gif <- animate(a, width = 240, height = 240)

b <- ggplot(stats_df, aes(x = ts))+
  geom_vline(aes(xintercept = ts, frame = sims))+
  geom_line(aes(x=x,y=y),
            data = data.frame(x = seq(-5,5, .1),
                              y = dt(seq(-5,5, .1), df = 18))) +
  theme_classic() +
  ylab("density") +
  xlab("t value") +
  transition_states(
    states = sims,
    transition_length = 2,
    state_length = 1
  ) +
  enter_fade() + 
  exit_shrink() +
  ease_aes('sine-in-out')

b_gif <- animate(b, width = 240, height = 240)

a_mgif <- image_read(a_gif)
b_mgif <- image_read(b_gif)

new_gif <- image_append(c(a_mgif[1], b_mgif[1]))
for(i in 2:100){
  combined <- image_append(c(a_mgif[i], b_mgif[i]))
  new_gif <- c(new_gif, combined)
}

new_gif

When I check the length of a_mgif and b_mgif, I get the following:

> a_mgif %>% length()
[1] 71
> b_mgif %>% length()
[1] 62

It seems that image_read() is truncating the number of frames in the gifs (which should be 100). I have tried fixing this by manually setting the number of frames of the gif to 100 in the animate() function, and have tried the coalesce = FALSE argument within image_read() to no avail. Any advice?

> sessionInfo()
R version 4.1.2 (2021-11-01)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 22.04.2 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.10.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.10.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8   
 [6] LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] gganimate_1.0.8 magick_2.7.4    ggplot2_3.4.2   dplyr_1.1.2    

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.10       rstudioapi_0.14   magrittr_2.0.3    hms_1.1.3         progress_1.2.2    tidyselect_1.2.0  munsell_0.5.0     colorspace_2.1-0 
 [9] R6_2.5.1          rlang_1.1.1       fansi_1.0.4       tools_4.1.2       grid_4.1.2        gtable_0.3.3      utf8_1.2.3        gifski_1.12.0-1  
[17] cli_3.6.1         withr_2.5.0       tibble_3.2.1      lifecycle_1.0.3   crayon_1.5.2      farver_2.1.1      tweenr_2.0.2      vctrs_0.6.3      
[25] glue_1.6.2        labeling_0.4.2    stringi_1.7.12    compiler_4.1.2    pillar_1.9.0      prettyunits_1.1.1 generics_0.1.3    scales_1.2.1     
[33] pkgconfig_2.0.3

ropensci / magick

image_read() dropping frames #373