does tabset not work at the h4 level?

Perhaps related to this issue, but I think different enough to merit a new issue:

I'm struggling to add a tabset at the h4 granularity. Take a look at this code:

    ---
    title: "report x"
    output: 
      flexdashboard::flex_dashboard:
        orientation: rows
        vertical_layout: fill
    ---
    
    
    ```{r setup, include=FALSE}
    suppressPackageStartupMessages(library(flexdashboard))
    library(htmltools)
    ```
    
    # page1
    
    ```
    
    ## Row 2
    
    ### Chart 4 {.tabset .tabset-fade}
    
    ```{r}
    ```
    
    #### Subchart 4.1
    
    ```{r}
    ```   
     
    #### Subchart 4.2
        
    ```{r}
    ```
    
    ## Row 3 {.tabset .tabset-fade}
    
    ### Chart 5
    
    ```{r}
    ```
    
    ### Chart 6
    
    ```{r}
    ```

Which creates the following (Note Figure 4 isn't obeying the tabset):

Here, you can see that even though Figure 4 is allowed to consume the entire row (as is the case w/ the h3 around it), the tabset still doesn't work. In trying to figure out the limitations around tabset, is it the case that h4 entities will never work with the tabset?

Start by rendering the reproducible flexdashboard example in the issue and compare the generated h3 and h4 sections. Inspect how the tabset behavior differs between those headings. Done means determining whether h4 tabsets are supported and documenting or correcting the observed behavior.