Tabset {.tabset} not working

First time I've experienced this problem. Have un/re-installed R, Rstudio multiple times, which does not seem to work.

---
title: "Missing Tabs"
output: html_document
---

## Tabset here {.tabset} 

### tab1

```{r}
summary(cars)
```

### tab 2

```{r}
summary(pressure)
```

Produces a document with no tabsets:

All packages are up to date, I've tried installing the latest dev version of rmarkdown using the remotes package, but the same problem occurs. Not sure what to do here at this point.

Session info:

> sessionInfo()
R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18362)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

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

other attached packages:
[1] rmarkdown_2.3

loaded via a namespace (and not attached):
 [1] rstudioapi_0.11   knitr_1.29        R6_2.4.1          rlang_0.4.7       fansi_0.4.1      
 [6] tools_4.0.2       pkgbuild_1.0.8    xfun_0.15         cli_2.0.2         withr_2.2.0      
[11] htmltools_0.5.0   remotes_2.1.1     yaml_2.2.1        digest_0.6.25     assertthat_0.2.1 
[16] rprojroot_1.3-2   crayon_1.3.4      processx_3.4.3    callr_3.4.3       ps_1.3.3         
[21] curl_4.3          glue_1.4.1        evaluate_0.14     compiler_4.0.2    backports_1.1.8  
[26] prettyunits_1.1.1
> devtools::session_info()
- Session info ------------------------------------------------------------------------------------
 setting  value                       
 version  R version 4.0.2 (2020-06-22)
 os       Windows 10 x64              
 system   x86_64, mingw32             
 ui       RStudio                     
 language (EN)                        
 collate  English_United States.1252  
 ctype    English_United States.1252  
 tz       America/New_York            
 date     2020-07-11                  

- Packages ----------------------------------------------------------------------------------------
 package     * version date       lib source        
 assertthat    0.2.1   2019-03-21 [1] CRAN (R 4.0.2)
 backports     1.1.8   2020-06-17 [1] CRAN (R 4.0.2)
 callr         3.4.3   2020-03-28 [1] CRAN (R 4.0.2)
 cli           2.0.2   2020-02-28 [1] CRAN (R 4.0.2)
 crayon        1.3.4   2017-09-16 [1] CRAN (R 4.0.2)
 curl          4.3     2019-12-02 [1] CRAN (R 4.0.2)
 desc          1.2.0   2018-05-01 [1] CRAN (R 4.0.2)
 devtools      2.3.0   2020-04-10 [1] CRAN (R 4.0.2)
 digest        0.6.25  2020-02-23 [1] CRAN (R 4.0.2)
 dplyr         1.0.0   2020-05-29 [1] CRAN (R 4.0.2)
 ellipsis      0.3.1   2020-05-15 [1] CRAN (R 4.0.2)
 evaluate      0.14    2019-05-28 [1] CRAN (R 4.0.2)
 fansi         0.4.1   2020-01-08 [1] CRAN (R 4.0.2)
 fs            1.4.2   2020-06-30 [1] CRAN (R 4.0.2)
 generics      0.0.2   2018-11-29 [1] CRAN (R 4.0.2)
 glue          1.4.1   2020-05-13 [1] CRAN (R 4.0.2)
 htmltools     0.5.0   2020-06-16 [1] CRAN (R 4.0.2)
 knitr         1.29    2020-06-23 [1] CRAN (R 4.0.2)
 lifecycle     0.2.0   2020-03-06 [1] CRAN (R 4.0.2)
 magrittr      1.5     2014-11-22 [1] CRAN (R 4.0.2)
 memoise       1.1.0   2017-04-21 [1] CRAN (R 4.0.2)
 pillar        1.4.6   2020-07-10 [1] CRAN (R 4.0.2)
 pkgbuild      1.0.8   2020-05-07 [1] CRAN (R 4.0.2)
 pkgconfig     2.0.3   2019-09-22 [1] CRAN (R 4.0.2)
 pkgload       1.1.0   2020-05-29 [1] CRAN (R 4.0.2)
 prettyunits   1.1.1   2020-01-24 [1] CRAN (R 4.0.2)
 processx      3.4.3   2020-07-05 [1] CRAN (R 4.0.2)
 ps            1.3.3   2020-05-08 [1] CRAN (R 4.0.2)
 purrr         0.3.4   2020-04-17 [1] CRAN (R 4.0.2)
 R6            2.4.1   2019-11-12 [1] CRAN (R 4.0.2)
 remotes       2.1.1   2020-02-15 [1] CRAN (R 4.0.2)
 rlang         0.4.7   2020-07-09 [1] CRAN (R 4.0.2)
 rmarkdown   * 2.3     2020-06-18 [1] CRAN (R 4.0.2)
 rprojroot     1.3-2   2018-01-03 [1] CRAN (R 4.0.2)
 rstudioapi    0.11    2020-02-07 [1] CRAN (R 4.0.2)
 sessioninfo   1.1.1   2018-11-05 [1] CRAN (R 4.0.2)
 testthat      2.3.2   2020-03-02 [1] CRAN (R 4.0.2)
 tibble        3.0.3   2020-07-10 [1] CRAN (R 4.0.2)
 tidyselect    1.1.0   2020-05-11 [1] CRAN (R 4.0.2)
 usethis       1.6.1   2020-04-29 [1] CRAN (R 4.0.2)
 vctrs         0.3.1   2020-06-05 [1] CRAN (R 4.0.2)
 withr         2.2.0   2020-04-20 [1] CRAN (R 4.0.2)
 xfun          0.15    2020-06-21 [1] CRAN (R 4.0.2)
 yaml          2.2.1   2020-02-01 [1] CRAN (R 4.0.0)

Also created a post over at SO: https://stackoverflow.com/questions/62856103/tabsets-are-not-rendering-in-knitted-rmd-files

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