failure in using tibble()

library(tidyverse)

a <- c(0, 1, 0)
M <- 2

tibble(
  a=rep(a, M),
  b=rep(1:M, each=length(a))
)

 エラー: Tibble columns must have compatible sizes.
* Size 6: Existing data.
* Size 12: Column `b`.
ℹ Only values of size one are recycled.
Run `rlang::last_error()` to see where the error occurred.

a <- c(0, 1, 0)
M <- 2

tibble(
  aa=rep(a, M), # changing this name 
  b=rep(1:M, each=length(a))
)

aa <- c(0, 1, 0) # a to aa
M <- 2

tibble(
  a=rep(aa, M),
  b=rep(1:M, each=length(aa)) # a to aa
)

a <- c(0, 1, 0)
M <- 2

tibble(
  a=rep(a, M),
  bb=rep(1:M, each=length(a)) # b to bb
)

 エラー: Tibble columns must have compatible sizes.
* Size 6: Existing data.
* Size 12: Column `bb`.
ℹ Only values of size one are recycled.
Run `rlang::last_error()` to see where the error occurred.

a <- c(0, 1, 0)
M <- 2

tibble(
  a=rep(a, M),
  b=rep(1:M, each=3) # = length(a), but ok in this case
)

a <- c(0, 1, 0)
M <- 2

list(
  a=rep(a, M),
  b=rep(1:M, each=length(a))
) %>% as_tibble()

sessionInfo()

R version 3.6.3 (2020-02-29)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.5 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.7.1
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.7.1

locale:
 [1] LC_CTYPE=ja_JP.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=ja_JP.UTF-8        LC_COLLATE=ja_JP.UTF-8    
 [5] LC_MONETARY=ja_JP.UTF-8    LC_MESSAGES=ja_JP.UTF-8   
 [7] LC_PAPER=ja_JP.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=ja_JP.UTF-8 LC_IDENTIFICATION=C       

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

other attached packages:
[1] forcats_0.5.1   stringr_1.4.0   dplyr_1.0.6     purrr_0.3.4    
[5] readr_1.4.0     tidyr_1.1.3     tibble_3.1.2    ggplot2_3.3.5  
[9] tidyverse_1.3.1

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.7           lubridate_1.7.10     prettyunits_1.1.1   
 [4] ps_1.6.0             assertthat_0.2.1     digest_0.6.27       
 [7] utf8_1.2.1           V8_3.4.2             R6_2.5.0            
[10] cellranger_1.1.0     backports_1.2.1      reprex_2.0.0        
[13] stats4_3.6.3         evaluate_0.14        httr_1.4.2          
[16] pillar_1.6.2         rlang_0.4.11         curl_4.3.1          
[19] readxl_1.3.1         rstudioapi_0.13      jquerylib_0.1.4     
[22] callr_3.7.0          rmarkdown_2.8        loo_2.4.1           
[25] munsell_0.5.0        broom_0.7.6          compiler_3.6.3      
[28] modelr_0.1.8         xfun_0.23            rstan_2.21.2        
[31] pkgconfig_2.0.3      pkgbuild_1.2.0       htmltools_0.5.1.1   
[34] tidyselect_1.1.1     gridExtra_2.3        codetools_0.2-18    
[37] matrixStats_0.58.0   fansi_0.4.2          crayon_1.4.1        
[40] dbplyr_2.1.1         withr_2.4.2          grid_3.6.3          
[43] jsonlite_1.7.2       gtable_0.3.0         lifecycle_1.0.0     
[46] DBI_1.1.1            magrittr_2.0.1       StanHeaders_2.21.0-7
[49] scales_1.1.1         RcppParallel_5.1.4   stringi_1.6.2       
[52] cli_2.5.0            fs_1.5.0             bslib_0.2.5.1       
[55] xml2_1.3.2           ellipsis_0.3.2       generics_0.1.0      
[58] vctrs_0.3.8          tools_3.6.3          glue_1.4.2          
[61] hms_1.0.0            processx_3.5.2       parallel_3.6.3      
[64] yaml_2.2.1           inline_0.3.17        colorspace_2.0-1    
[67] rvest_1.0.0          knitr_1.33           haven_2.4.1         
[70] sass_0.4.0