Help me understand Heartworm Disease

I help run a rescue called in the Maryland area, we rescue dogs from Puerto Rico. Given the climate, mosquitos carrying heartworm are very common. We are DILIGENT about preventatives and give Nexgard Spectra Plus. Puppies under six months do not get a 4dx test. It's my understanding that if a dog is carrying heartworm larvae, the Nexgard will prevent them from turning into adult heartworms. Any dog that is over six months gets a 4dx before flying, since treating HW is MUCH cheaper on the island.

Here's the issue: we are still seeing what I consider a high number of heartworm positive cases in dogs after they are adopted. One case was a puppy who was adopted and the owner never missed a dosage of preventative. Another was a senior dog who had a negative HW test in Puerto Rico and then came up positive here. This is very concerning to me.

Specific Questions: Is is true that Nexgard Spectra Plus will kill baby heartworms?

Are there different kind of Heartworm tests? Is it possible PR vets are using a less accurate type of test and if so what would be the correct question to ask? Can you suggest any type of protocol that would help us prevent these cases?

Is it possible that the tests here are detecting baby heartworms, and what further testing could be done to know?

Thank you so much for your time! I am really trying to get a full understanding to prevent more cases.