DFA and melee both completely ignore the stability-reducing effects of bulwark or brace, and in fact remove the entire bulwark/brace effect.
It's Evasion that is disrupted by enough instability (the exact amount required varies by pilot skill); the bulwark/brace effect is much more binary and is disabled by any and all physical attacks. A locust punch would disrupt it.
You're correct, my not-even-close-to-appropriately-caffeinated brain was incorrectly conflating the two.
To be clear though, you can still push enough instability via non-physical means to break a brace.