Replaceable templates not working across multiple levels

Replaceable templates accross multiple levels as shown in the following sketch are not working. The replaceable template in `tab.html` gets replaced with empty content.

tab.html
```html

```
tab-control.html
```html

```
app.html
```html

...

```
Neither works this:

tab.html
```html

```
tab-control.html
```html

```
app.html
```html

...

```

Start by reproducing the nested replaceable-template examples from tab.html, tab-control.html, and app.html, comparing both arrangements described in the issue. Trace how replaceable and replace-part content is resolved across the nested templates, then verify that the deepest form content appears in the tab instead of being replaced with empty content.

Written by the indexing model from the issue text.