Festival related: SIM BORDER CROSSING
You will fall thru the border between the STAGE and the AUDIENCE sims to the next level below it when walking across it.
So here is a solution:
Add a prim BELOW the platform surface that is 63 x 10 x 0.5 along the length of the crossing. You will dip just slightly but not fall thru. And remember to take it with you when you clean up, or if you leave it, know that it may be auto-returned by security. I am not sure what the policy will be.
And that's Yummy's *taste of things