Why does cur_data() within summarize() return df_slice() error?

Question

I ran into trouble today when using cur_data() within summarize().

Example data:

library(tidyverse)

dat <- tibble(id = 1:6,
              type = c(1, 1, 2, 2, 3, 3),
              value = c(2, 4, 6, 8, 7, NA))

This first pipeline throws an error, mentioning df_slice():

dat %>%
  group_by(type) %>%
  summarize(mean = mean(value),
            n = length(cur_data() %>% filter(!is.na(value)) %>% pull(id) %>% unique()),
            .groups = "drop")
#> Error in `summarize()`:
#> ! Problem while computing `n = length(...)`.
#> ℹ The error occurred in group 1: type = 1.
#> Caused by error:
#> ! Internal error in `df_slice()`: Columns must match the data frame size.

However, switching the order of the summary stats within summarize() avoids the error:

dat %>%
  group_by(type) %>%
  summarize(n = length(cur_data() %>% filter(!is.na(value)) %>% pull(id) %>% unique()),
            mean = mean(value),
            .groups = "drop")
#> # A tibble: 3 × 3
#>    type     n  mean
#>   <dbl> <int> <dbl>
#> 1     1     2     3
#> 2     2     2     7
#> 3     3     1    NA

Additionally, piping cur_data() into as.data.frame() also avoids the error:

dat %>%
  group_by(type) %>%
  summarize(mean = mean(value),
            n = length(cur_data() %>% as.data.frame() %>% filter(!is.na(value)) %>% pull(id) %>% unique()),
            .groups = "drop")
#> # A tibble: 3 × 3
#>    type  mean     n
#>   <dbl> <dbl> <int>
#> 1     1     3     2
#> 2     2     7     2
#> 3     3    NA     1
Created on 2022-02-15 by the reprex package (v2.0.1)

Why can I not use the first example syntax? Ultimately I calculated anything that required cur_data() within mutate() and just kept the first() observation within a later summarize() call, but I'd like to know what I'm missing about summarize().

Additional session info:

> sessionInfo()
R version 4.1.2 (2021-11-01)
Platform: aarch64-apple-darwin20.6.0 (64-bit)
Running under: macOS Monterey 12.1

Matrix products: default
LAPACK: /opt/homebrew/Cellar/r/4.1.2/lib/R/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

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

other attached packages:
 [1] reprex_2.0.1         palmerpenguins_0.1.0 forcats_0.5.1        stringr_1.4.0        readr_2.1.2         
 [6] tibble_3.1.6         ggplot2_3.3.5        tidyverse_1.3.1      tidyr_1.2.0          purrr_0.3.4         
[11] dplyr_1.0.8