Having a 911, weird issue with DNN 9.x.

I am building a series of custom page skins, and custom container skins --- all based on a really clean bootstrap template. I've done this countless times without issue.

I have a custom container (module) skin, called HeroSection.ascx. When the page/site is in edit mode, the styling/html elements within HeroSection.ascx appear as you'd expected. When the site/page is taken out of edit mode (returned to viewing), all of the styling/markup elements within HeroSection.ascx disappear -- the content of the module is still there, but the there's no trace of the container skin.

The container and skin files are in their correct places within the site structure. The page/container skins are set in Manage -> Themes

To test this further, I created an additional container skin called default.ascx (full code below), and added the "wtaf" ID so I could search for it in the browser's console. I applied the container skin and the red background shows up when the site/page is in edit mode. When I take the site/page out of edit mode, the module's content is there, but the red bar disappears and the container skin's markup (including the wtaf) is nowhere to be found in the browser console.

Any idea what's going on here? THANKS

[code/markup snippet is visible at the above StackOverflow link.  Sorry, couldn't figure out how to post it here]

Update: -doesn't work/matter if the container skin is placed within the /portal/[site-System] OR /portal/_default. In other words, doesn't matter if it's a host or site/portal skin.