That sort of depends on context. If the failure was reasonably expected, like for example your router is running on prototype underclocked hardware or is a debug build that is known not to have the final performance characteristics, sure. But if it fails and you don't immediately suspect why it failed, then that's a sign of a genuine design flaw.

And sure, it's good to find the design flaws in "dev" vs. "prod", but it's still bad to have them.

In this case, we don't really know what the expected performance of that flap shielding was. Maybe it was a kludge and they were just hoping the glue held together, maybe it was a finished design they just thought they were validating. The latter is a much (!) bigger problem than the former.