This works:

mosaic-headline.html

Code: Select all

<cms:mosaic name='ce' label='Content'>
    <cms:tile name='headline' label='Headline'>
        <cms:embed 'tile-headline.html' />
    </cms:tile>
</cms:mosaic>

tile-headline.html

Code: Select all

<cms:editable name='headline' label='Headline' type='text'/>

This does not work:
mosaic-headline.html

Code: Select all

<cms:mosaic name='ce' label='Content'>
        <cms:embed 'tile-headline.html' />
</cms:mosaic>

tile-headline.html

Code: Select all

<cms:tile name='headline' label='Headline'>
    <cms:editable name='headline' label='Headline' type='text'/>
</cms:tile>

I am not sure, if this is a bug

[KK]

No it is not a bug. Mosaic expects the tiles to be its direct children and ignores all other tags. That would explain your observation.
The cms:tile tag, however, accepts embeds. So, your first method is the correct way to go about things.