Never host your test environments as Subdomains of your actual production domain. You'll also run into email reputation as well as cookie hell. You can get a lot of cookies from the production env if not managed well.
This. I cannot believe the rest of the comments on this are seemingly completely missing the problem here & kneejerk-blaming Google for being an evil corp. This is a real issue & I don't feel like the article from the Immich team acknowledges it. Far too much passing the buck, not enough taking ownership.
Similarly, if an organization deploys a public system that engages in libel and tortious interference, the fact that jumping through technical hoops might make it less likely to be affected by that system does not in any way absolve the organization for operating it carelessly in the first place.
Just because there are steps you can take to lessen the impact of bad behavior does not mean that the behavior itself isn't bad. You shouldn't have restrict how you use your own domains to avoid someone else publishing false information about your site. Google should be responsible for mitigating false positives, not the website owners affected by them.