`ggalluvial`: `geom_flow` fill color from destination stratum

I have the following example:

library(tidyverse)
library(ggalluvial)
data <- tibble(id = paste0("S", 1:20), 
       class_1 = c(rep("A", 10), rep("B", 10)),
       class_2 = c(rep("A", 8), rep("B", 8), rep("A", 4)))
data_pvt <- data %>%
    pivot_longer(starts_with("class"), names_to = "class_type", values_to = "class_label") %>%
    mutate(class_type = factor(class_type),
           class_label = factor(class_label))
ggplot(data_pvt, aes(x = fct_rev(class_type), stratum = class_label, alluvium = id,
                     label = class_label)) +
    geom_flow(aes(fill = class_label), stat = "alluvium", 
              lode.guidance = "frontback") +
    geom_stratum(aes(fill = class_label)) +
    scale_x_discrete(expand = c(0.1, 0)) +
    labs(x = "Class system", y = "n") +
    coord_flip() +
    theme_minimal()

^{Created on 2022-02-18 by the reprex package (v2.0.1)}

I would like geom_flow to take the fill color from the top stratum (class_1) instead of the bottom stratum (class_2). I could achieve this by not fct_rev(class_type) at the beginning, but then class_1 is at the bottom, while I want it at the top.

Any ideas? I could use other functions from ggalluvium or ggforce, but I'd like to keep the option of having the stratum colored by class_label.

Are you looking for aes.flow = "backward"?

ggplot(data_pvt, aes(x = fct_rev(class_type), stratum = class_label, alluvium = id,
                     label = class_label)) +
    geom_flow(aes(fill = class_label), stat = "alluvium", 
              lode.guidance = "frontback", aes.flow = "backward") +
    geom_stratum(aes(fill = class_label)) +
    scale_x_discrete(expand = c(0.1, 0)) +
    labs(x = "Class system", y = "n") +
    coord_flip() +
    theme_minimal()