This time I caught a wild one: http://plnkr.co/edit/YCozy6N2pJ4qWgN24RjI?p=preview
What I think is happening is that 'mount' gets called before props are inherited, causing an inconsistent behavior.
Yup, here it is: https://github.com/riot/riot/blob/master/lib/browser/tag/each.js#L131
So it seems you should listen to 'update', not to 'mount', if you want to use inherited props.
This one works fine: http://plnkr.co/edit/4RP2fKUzRYhGhPh5KlcN?p=preview
From what I understand, the issue is simply that parent-tag has not added the entry to its list when mount() is called in child-tag. I don't think that this is a bug...
Here is another plunker illustrating this: http://plnkr.co/edit/L7HOge4F0t9N0K9kcmB8?p=preview
As I mentioned earlier, properties are inherited when parent tag updates, which happens after you mount the child tag.
I can fix this by moving this out of the update callback to the separate function and calling it both there and here
Note that this behavior will be completely removed in firstname.lastname@example.org #1697