[X] Catch up to them and ask them what they're doing.
HOWEVER, this also means that Reactive Purge faces a big problem: Detection. Unlike the reactive tank/mech armor the idea is based on, under the barrier jacket is flesh rather than a steel backing, so a reactive armor discharge needs space to repel a physical attack unless you want the reactive armor burst to kill the wearer even faster. So with the AMF degrading the outer field, it couldn't detect the incoming spike until it was already in contact, and that led to penetration.
The counter to that is thicker physical armor layers and an emphasis on reinforced material over field and reactive effects, which would translate fairly well to the much thicker designs of Strikers compared to younger versions having more gem/field emitters.
All headcanon of course
I believe the Field Effect defense counters the AMF normally, since it's basically an environmental effect. It would greatly explain how barrier jacket equipped mages only suffer a rank down in power: the barrier jacket shields the mage from the effects of the AMF, but the mana drain of maintaining a protective field effect against a magic negating effect would lower the output of possible spells.Now, that said, I do still have an issue with what I think your point actually is. A Barrier Jacket consists of two or three layers of protection - Field effects (primarily for environmental effects, AFAIK); Barrier effects (disperse and deflect most incoming attacks); and possibly reinforced physical material. There is also, at least in some cases, the Reactive Purge seen in Nanoha's first fight with Vita in A's, which appears to be a last-ditch effect unrelated to the normal defenses. An AMF disrupts spells cast within the field itself, as well as degrades spells cast from outside it, but in both cases you're still talking about high-level mages with active protections cast from outside the AMF, and Vita in particular would be much more familiar with AMF due to her history and the knowledge that she's actually going to be fighting inside one. So, yes, there would have been weakening of the Barrier Jackets' protections, but those should have been to the outer layers of protections first, and considering the strength and capabilities of the mages in question, it should not have been to such a degree that the Barrier Jackets lost core functionality.
There is, also, the curious fact that neither event triggered a Reactive Purge, which seems odd. My favored headcanon is that Reactive Purges were disabled during those encounters due to the nature of the territory - Nanoha, IIRC, was in a situation with opponents of unknown capabilities, and so depleting your defenses in such a manner was considered unwise; and Vita was going into an area where she would be outnumbered if any engagement occurred, at which point the same logic applies. The only other possibility I've come up with is that the Reactive Purges didn't trigger because the Type IV Gadget Drones attacked by surprise, but since a large point of automated and reactive defenses like Barrier Jackets and Reactive Purges is that the user doesn't have to know about the incoming attack, that doesn't seem likely. Possible, since even with the ambiguity of magic and what it can do, MGLN still seems to try to keep things realistic, so unpredictable events may simply be too difficult to account for with the programming of Devices, but it's not one I'm really attached to.
HOWEVER, this also means that Reactive Purge faces a big problem: Detection. Unlike the reactive tank/mech armor the idea is based on, under the barrier jacket is flesh rather than a steel backing, so a reactive armor discharge needs space to repel a physical attack unless you want the reactive armor burst to kill the wearer even faster. So with the AMF degrading the outer field, it couldn't detect the incoming spike until it was already in contact, and that led to penetration.
The counter to that is thicker physical armor layers and an emphasis on reinforced material over field and reactive effects, which would translate fairly well to the much thicker designs of Strikers compared to younger versions having more gem/field emitters.
All headcanon of course