Many rsessions mostly idle with `parallelism = "future"`

Prework

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Before trying to create a reprex, I wanted to make sure there is no obvious short answer. If this is not the case, I'll try to create a reprex.

Description

I noticed that very often, many of the rsessions which should be used for parallel processing in my drake plan, are idle (see screenshot).

I'm sure that the general setup with future::plan() before drake::make is correct, because some targets really run in parallel as I expect.

Is there a possibility that this is related to setting format = "file" for dynamic targets? Whenever I noticed that only one session is actually active, drake was working on a dynamic target with format = "file".

Desired result

All dynamic subtargets should run in parallel.

Session info

```r > sessionInfo() R version 4.0.2 (2020-06-22) Platform: x86_64-w64-mingw32/x64 (64-bit) Running under: Windows 10 x64 (build 18363) Matrix products: default locale: [1] LC_COLLATE=German_Germany.1252 LC_CTYPE=German_Germany.1252 LC_MONETARY=German_Germany.1252 LC_NUMERIC=C [5] LC_TIME=German_Germany.1252 attached base packages: [1] stats graphics grDevices datasets utils methods base other attached packages: [1] knitr_1.30 arrow_4.0.0 lubridate_1.7.9 haven_2.4.0 readxl_1.3.1 fs_1.5.0 anytime_0.3.9 [8] stringi_1.5.3 purrr_0.3.4 data.table_1.14.0 drake_7.12.7 dobby_0.1.0.111 conflicted_1.0.4 loaded via a namespace (and not attached): [1] Rcpp_1.0.5 txtq_0.2.3 listenv_0.8.0 visNetwork_2.0.9 prettyunits_1.1.1 ps_1.4.0 [7] assertthat_0.2.1 digest_0.6.27 R6_2.5.0 cellranger_1.1.0 backports_1.2.0 evaluate_0.14 [13] pillar_1.4.7 rlang_0.4.10 settings_0.2.4 progress_1.2.2 rstudioapi_0.13 callr_3.5.1 [19] R.utils_2.10.1 R.oo_1.24.0 DT_0.16 checkmate_2.0.0 rmarkdown_2.5 qs_0.24.1 [25] stringr_1.4.0 htmlwidgets_1.5.2 igraph_1.2.6 bit_4.0.4 validate_0.9.3 compiler_4.0.2 [31] xfun_0.19 pkgconfig_2.0.3 askpass_1.1 globals_0.13.1 htmltools_0.5.0 openssl_1.4.3 [37] tidyselect_1.1.0 tibble_3.0.4 codetools_0.2-16 future_1.19.1 crayon_1.3.4 R.methodsS3_1.8.1 [43] jsonlite_1.7.1 lifecycle_0.2.0 magrittr_2.0.1 storr_1.2.4 RcppParallel_5.0.2 cli_2.5.0 [49] cachem_1.0.4 renv_0.13.2 ellipsis_0.3.1 filelock_1.0.2 vctrs_0.3.5 generics_0.1.0 [55] stringfish_0.15.1 fst_0.9.4 RApiSerialize_0.1.0 tools_4.0.2 forcats_0.5.0 bit64_4.0.5 [61] glue_1.4.2 hms_0.5.3 processx_3.4.4 parallel_4.0.2 fastmap_1.0.1 yaml_2.2.1 [67] base64url_1.4 memoise_2.0.0 ```

ropensci / drake