In case someone who use Subscription in strapi graphql run into same problem, It is a problem of nexus where strapi graphql plugin used mergeSchema. You can fix it in local workspace (refer: fix nexus mergeSchema subscribe return undefined).
I also update a subscription example in the case repo, strapi-study-cases/src/plugins/graphql at main · GavinXue/strapi-study-cases · GitHub
When I try to find the reason where cause it. I have change apolloServer to v4, and use graphql-ws to offer subscription, it seems work in my dev environment. (maybe update to case repo later)