my guess is that inbreeding can cause issues, but not as often as we may think
the reason inbreeding causes issues is because it allows lethal recessives to accumulate, but there are a few issues with this. 1: it assumes there are lethal recessives to begin with. I'm sure that in SOME cases, there are, but I doubt it happens every damn time. The other issue is that continual inbreeding flushes these recessives out entirely
Keep in mind a whole bunch of parasitoid wasp species have a life history where the mother lays a bunch of female and a few male eggs into a host, the larvae develop, and immediately upon emerging, the male inseminates his sisters. These are some of the most diverse wasps in the world
we also heavily inbreed basically all feeder species, including mice. I suspect issues with mantis breeding occur because of bad diet, lack of UV light, improper temperatures, bacterial infections, etc
I know that I have had far fewer issues with mantis rearing and breeding in general when I started cleaning all of my rearing supplies with 10% bleach, gut loading feeders with highly nutritious food (including crested gecko diet), offering varied diet, and having day/night temperature shifts