Cloudflare Workers often rolls out V8 security patches to production before Chrome itself does. That's different from beta vs. stable channel. When there is a security patch, generally all branches receive the patch at about the same time.

As for beta vs. stable, Cloudflare Workers is generally somewhere in between. Every 6 weeks, Chrome and V8's dev branch is promoted to beta, beta branch to stable, and stable becomes obsolete. Somewhere during the six weeks between verisons, Cloudflare Workers moves from stable to beta. This has to happen before the stable version becomes obsolete, otherwise Workers would stop receiving security updates. Generally there is some work involved in doing the upgrade, so it's not good to leave it to the last moment. Typically Workers will update from stable to beta somewhere mid-to-late in the cycle, and then that beta version subsequently becomes stable shortly thereafter.

(I'm the lead engineer for Cloudflare Workers.)