@smithjd can you share the website where this happens ? Can you share some Rmd code that I can run ?
Also does it works correctly in a html_document() ouput format ?
From a quick test using
---
title: "Untitled"
output:
html_document: default
distill::distill_article: default
editor_options:
markdown:
canonical: false
---
# TABLE
<details><summary>Table content</summary>
```{r, echo = FALSE}
DT::datatable(mtcars)
```
</details>
it seems with both format Firefox does not update the details tag content on open. On CHROME it is working fine. It does not seem to be specific to distill
Unfortunately, it seems to be a browser issue. It is as if Chrome 'reload' correctly the page as with Firefox, if I click to open and then open the inspector window, it will show correctly.
Anyway, I am not sure what we could do on our side for that 
Happy to take any idea.