The lockout happens if a player respawns while still scrambled, i.e. only when the remaining scramble duration at the time of death exceeds the respawn timer of 15s and the player presses the respawn button before the scramble runs out.
Easy ways to avoid the bug:
1) Don't respawn while you still have the SS icon counting down.
2) Spec into sensors. Then only the most unfortunate circumstances (full aux + full spec SS3 + death immediately after being hit by SS) have a chance of giving you an SS timer that is longer than the respawn timer. And in these rare cases apply 1).