Not withstanding the debate about whether a fracture could truly occur, the question here is centered around why the human bomb sent in advance of the raiding party did not cause such a fracture to occur according to the rules expressed in-Universe [which characters seem to believe, even if wrongly].
The ability to collapse the space-time fracture, requires the trigger (explosion, sonic device, etc) to occur, at the very least, on the side of portal that's transmitting. Whether there's a requirement that the portal must be operating in a bi-directional capacity (i.e, matter able to traverse both directions at the same time) is unknown. However, we can presume that the danger presented in the scene between Lucas and Taylor was a new development for Terra Nova because the portal was reversed and the terminus was transmitting rather than receiving. It could be that this general knowledge/fear/theory was always a point of concern and viable danger in their original timeline (Hope Plaza side).
Why didn't the fracture collapse when the human bomb came through and went boom?
At that point in time, Terra Nova was receiving a standard pilgrimage. There's no reason to believe that the terminus in TN was also transmitting. The terminus was operating on TN side in the same capacity it had prior to Lucas' discovery, therefore no blow back through the fracture. It was 'innert' so to speak and destroying it would only have unanchored it from the fracture.