Sylvan primordial destroying detention sphere that previously exiled primeval bounty,
Let's look at the stack:


We play Sylvan Primordial.
Sylvan Primordial resolves, and is removed from the stack.
Sylvan Primordial's 'Enters the battlefield' ability is placed on the stack. You choose Detention Sphere as the target.
Sylvan Primordial's triggered ability resolves. Detention Sphere is destroyed, and you may search your library for 1 Forest and put it into play tapped. 
Because the Detention Sphere was destroyed, its 'Leaves the battlefield' ability is now placed on the stack.
Detention Sphere's triggered ability resolves. 'Return previously exiled Primeval Bounty to play' - The forest is already in play, so it doesn't trigger Primeval Bounty's ability.


Because you might assume that there's a timing difference between the destroying of permanents and the counting of those permanents for the forest-retrieval effect, it's logical to think that the consequences of said destruction will have already resolved. This is not the case however, as is shown by Sylvan Primordial's specific Ruling on the Gatherer:


  If Sylvan Primordial’s enters-the-battlefield ability resolves but one of the target noncreature permanents isn’t destroyed (perhaps because it has indestructible or it regenerated), it won’t count toward the number of Forest cards you can put onto the battlefield.


Its ETB trigger resolves in one motion, and the additional ruling compensates for the difference in targets chosen vs targets saved by opponents.