Why is this. Aren't all bounty questions supposed to be visible there?
As indicated over here, the sorting of the Featured tab is by the time left.
Since you posted the bounty at 16:09 UTC and it has only been roughly 2 hours since then, it still has a lot of time left. As of writing this answer, I found it on the last page of the full list, near the bottom but not at it.
The front page sort, unlike the Questions list, is cut short when it reaches the maximum for one page. You'll find at the bottom the following line:
Looking for more featured questions? Browse the complete list.
As such, as long as you aren't on the first page of featured questions, you won't be on the front page listing. You'll still be present in the full listing, as well as on the short lists of anyone who search by tags.