Matrix heatmap in ggplot not consistent in upper and lower triangles

Question

For some reason, this symmetrized matrix is plotting with either no data in the lower triangle of the heatmap or just a little as shown in the below figure. I can't figure out why it's happening. Any guesses? I've shared the first 100 rows of the dataset in the example below.

I'm using the following:

library(igraph)
library(dplyr)
library(ggplot2)
library(viridis)
Edgelist = "from  to    weight
2D16  31C1       3
2D16  3215       4
2D16  3AFD       3
2D16  3CFF       3
2D16  4777       2
2D16  48E2       3
2D16  4BAA       3
2D16  4FBB       3
2D16  5323       1
2D16  5A68       4
2D16  5D78       4
2D16  5D9E       4
2D16  5E0F       1
2D16  6BF3       2
2D16  6DBC       2
2D16  6EFC       2
2D16  6FB0       4
2D16  714B       3
2D16  735B       1
2D16  7E8A       3
2D16  7F77       1
2D16  8761       4
2D16  82D8       3
2D16  8BE7       2
2D16  8D8C       4
2D16  90B1       1
2D16  9116       1
2D16  9264       3
2D16  99CB       3
2D16  9A69       3
2D16  A26F       3
2D16  A975       3
2D16  AC37       2
2D16  B190       3
2D16  B261       2
2D16  B6DE       3
2D16  B7FD       1
2D16  BAB5       4
2D16  BDB5       1
2D16  C044       2
2D16  C1D9       4
2D16  C400       2
2D16  C574       2
3011  3011       3
3011  31C1       3
3011  3215       3
3011  3AFD       3
3011  3CFF       3
3011  4777       2
3011  48E2       3
3011  4BAA       3
3011  4FBB       3
3011  5323       3
3011  55E2       1
3011  5594       2
3011  5A68       3
3011  5D78       3
3011  5D9E       2
3011  6155       1
3011  6BF3       3
3011  6EFC       1
3011  6FB0       3
3011  714B       2
3011  7E8A       3
3011  843C       1
3011  8761       3
3011  82D8       3
3011  8BE7       1
3011  8D8C       3
3011  90B1       2
3011  9116       1
3011  9264       1
3011  99CB       2
3011  9A69       3
3011  A26F       3
3011  A975       3
3011  AC37       3
3011  B190       1
3011  B261       3
3011  B7FD       2
3011  BAB5       3
3011  BDB5       1
3011  C1D9       1
3011  C574       3
31C1  2D16       3
31C1  3011       3
31C1  31C1      15
31C1  3215      15
31C1  3AFD      15
31C1  3CFF      13
31C1  4777      10
31C1  48E2      15
31C1  4BAA      15
31C1  4FBB      14
31C1  5323       9
31C1  55E2       1
31C1  5594       4
31C1  5A68      13
31C1  5D78      12
31C1  5D9E      13"
Data <- read.table(text=Edgelist, header = TRUE)
graph <- graph.data.frame(Data, directed = FALSE)

V(graph)$comm <- membership(optimal.community(graph))
node_list <- get.data.frame(graph, what = "vertices")
edge_list <- get.data.frame(graph, what = "edges") %>%
  inner_join(node_list %>% select(name, comm), by = c("from" = "name")) %>%
  inner_join(node_list %>% select(name, comm), by = c("to" = "name")) %>%
  mutate(group = ifelse(comm.x == comm.y, comm.x, NA) %>% factor())
all_nodes <- sort(node_list$name)

plot_data <- edge_list %>% mutate(
  to = factor(to, levels = all_nodes),
  from = factor(from, levels = all_nodes))

  ggplot(plot_data, aes(x = from, y = to, fill = weight)) +
       geom_tile() +
       theme_bw() +
       scale_x_discrete(drop = FALSE) +
       scale_y_discrete(drop = FALSE) +
       theme(axis.title.x=element_blank(),
            axis.title.y=element_blank(),
            axis.text.x = element_text(angle = 270, hjust = 0),
            aspect.ratio = 1,
            legend.position = "bottom") +  
              scale_color_viridis(discrete = FALSE, option = "D") +
              scale_fill_viridis(discrete = FALSE)