How do I integrate dynamic files into drake's cache?

Question

I would like to use dynamic files for my drake plan. I have followed drake's documentation on dynamic files.

However, drake does not seem to incorporate the file paths created into its cache.

In the following reprex, plan_a follows the documentation, but ends up rebuilding the targets figure_export_path and hence figure_export on every run. plan_b does not rebuild anything on the second run, but its code is inconsistent with drake's documentation.

What is the correct way to use dynamic files in a situation like mine?

library(drake)
library(tidyverse)

foo <- function(bar) {
  y <- pmax(1:3, bar)
  qplot(x = 1:3, y = y, geom = "point")
}

plan_a <- drake::drake_plan(
  parameter = 1:3,

  figure = target(
    foo(parameter),
    dynamic = map(parameter)
  ),

  figure_export_path = target(
    paste(parameter, 'pdf', sep = '.'),
    dynamic = map(parameter),
    format = "file" # This line seems to invalidate the cache
  ),
  
  figure_export = target(
    ggsave(figure_export_path, figure),
    dynamic = map(figure_export_path, figure)
  )
)

plan_b <- drake::drake_plan(
  parameter = 1:3,
  
  figure = target(
    foo(parameter),
    dynamic = map(parameter)
  ),
  
  figure_export_path = target(
    paste(parameter, 'pdf', sep = '.'),
    dynamic = map(parameter),
    # format = "file" # This line seems to invalidate the cache
  ),
  
  figure_export = target(
    ggsave(figure_export_path, figure),
    dynamic = map(figure_export_path, figure)
  )
)

rerun(2, drake::make(plan = plan_a))
#> ▶ target parameter
#> ▶ dynamic figure
#> > subtarget figure_0b3474bd
#> > subtarget figure_b2a5c9b8
#> > subtarget figure_71f311ad
#> ■ finalize figure
#> ▶ dynamic figure_export_path
#> > subtarget figure_export_path_0b3474bd
#> Warning: missing dynamic files for target figure_export_path_0b3474bd:
#>   1.pdf
#> > subtarget figure_export_path_b2a5c9b8
#> Warning: missing dynamic files for target figure_export_path_b2a5c9b8:
#>   2.pdf
#> > subtarget figure_export_path_71f311ad
#> Warning: missing dynamic files for target figure_export_path_71f311ad:
#>   3.pdf
#> ■ finalize figure_export_path
#> ▶ dynamic figure_export
#> > subtarget figure_export_25d05a3f
#> Saving 7 x 5 in image
#> > subtarget figure_export_4e67ff77
#> Saving 7 x 5 in image
#> > subtarget figure_export_5ac7bd49
#> Saving 7 x 5 in image
#> ■ finalize figure_export
#> ▶ dynamic figure_export_path
#> > subtarget figure_export_path_0b3474bd
#> > subtarget figure_export_path_b2a5c9b8
#> > subtarget figure_export_path_71f311ad
#> ■ finalize figure_export_path
#> ▶ dynamic figure_export
#> > subtarget figure_export_e284629c
#> Saving 7 x 5 in image
#> > subtarget figure_export_7278fc00
#> Saving 7 x 5 in image
#> > subtarget figure_export_72064f81
#> Saving 7 x 5 in image
#> ■ finalize figure_export
#> [[1]]
#> NULL
rerun(2, drake::make(plan = plan_b))
#> ▶ dynamic figure_export_path
#> > subtarget figure_export_path_0b3474bd
#> > subtarget figure_export_path_b2a5c9b8
#> > subtarget figure_export_path_71f311ad
#> ■ finalize figure_export_path
#> ▶ dynamic figure_export
#> > subtarget figure_export_5a8b1281
#> Saving 7 x 5 in image
#> > subtarget figure_export_05ed8067
#> Saving 7 x 5 in image
#> > subtarget figure_export_3c33e0b9
#> Saving 7 x 5 in image
#> ■ finalize figure_export
#> ✓ All targets are already up to date.
#> [[1]]
#> NULL

^{Created on 2020-08-03 by the reprex package (v0.3.0)}