Does Riptide Rex bypass Deflect?

No, Riptide Rex does not bypass Deflect.

Riptide Rex's triggered ability ("When you play me, deal 6 to an enemy unit at a battlefield") chooses a target — it selects an enemy unit. Because it chooses, Deflect applies normally, adding a mandatory Power cost to the ability (Rule 809.1.c).

However, since this is a triggered ability (not a spell), you may decline to pay the Deflect cost. If you decline, the ability fails to finalize on the Chain and is removed without dealing damage (FAQ #8661, confirmed by Patch Notes 2025-10-24). If you do pay the cost, the ability resolves and deals 6 damage as normal.

This is different from abilities like Vex, Apathetic or Whirlwind, which use programmatic selection and don't choose targets at all — those genuinely bypass Deflect.