I've looked into this too and think it may be a poorly implemented feature, and not a "bug". My reason for thinking this?
Without even using spawn, if you place in a ship within visual range of your boat scheduled to appear 1 minute after the mission begins, it will not appear. However, Navel Bases can appear after your mission starts. So possibly there is some mechanism by which having a "ghost ship" to appear before your very eyes is not allowed.
On the otherhand, the delete feature is unreliable also, not disabled but not right, either. (If I remember righ I tried to get ship 1 to dissapear when ship 2 was sunk, but ship 2 was the one that dissappears and for no apparent reason)
Incidently, I've also had a lot of trouble with the time trigger. If you schedule a radio message to appear more than a few hours in the future, it will appear as soon as the mission starts instead.
