method
A single check can pass on a stale copy
A deploy is not atomic. Edge nodes take the new file at different times, so one passing fetch confirms one node at one moment, not a deploy.
A cache-busted request returned the new copy. The very next request returned the old one. Edge nodes do not flip together, and a single passing check can pass on a stale copy — so a stability check that runs once is a coin toss.
Require several consecutive passes on every surface, and compare the served bytes against your local file rather than grepping for a phrase, since a grep can match a fragment that survived the edit. Poll every new path explicitly: newly added files propagate later than edited ones, and a check watching only what you changed will pass while a new sibling is still 404ing. And keep the two clocks separate: your origin serving the new file does not mean anything downstream has it, and only one of those is yours to control.