A site owner on Reddit said Google brought back their old domain four months after a migration that looked complete, leaving the new domain with around 20 impressions a week. Google's John Mueller replied that some migrations take a long time to fully process. He advised setting up Search Console's Change of Address for both the www and non-www versions of the domain.
A migration isn't done when traffic comes back. This case shows Google can swing back months later, so a launch-week check is not enough.
For Shopify brands changing domain or replatforming, the boring details decide the outcome: redirects that keep each URL path, no chains, internal links updated, a sitemap on the new domain, and Change of Address on both www and non-www.
We would plan at least six months of monitoring after any move, and keep the old domain and its redirects in place well beyond that.