There's a very large gulf between "what makes sense to Google" and "what makes sense to Human Beings". I have so many rants about Google's poor treatment of "customers" that they feel like Oracle to me now. Like every time I use them, I'm really just falling prey to my own misguided idea that this time I won't get screwed over.
The users aren't random "human beings" in this case. They are professional software developers who are expected to understand the basics. Deploying that model into production shows a lack of basic competence. It is clearly marked "preview" and is for test only.
That may be true, but it doesn't make the customer's claims not true. What Google did was counter-intuitive. That's a fact. Pointing at some fine print and saying "uhh actually, technically it's your stupid human brain is the problem, not us! we technically are allowed to do anything we want, just look at the fine print!!" does not make things better. We are human beings; we are flawed. That much should be obvious to any human organization. If you don't know how to make things that don't piss off human beings, the problem isn't with the humans.
If the "preview release" you were using was v0.3, and suddenly it started being v0.6 without warning, that would be insane. The only point of providing a version number is to give people an indicator of consistency. The datestamp is a version number. If they didn't want us to expect consistency, they should not have given it a version number. That's the whole point of rolling release branches, they have no version. You don't have "v2.0" of a rolling release, you just have "latest". They fucked up by giving it a datestamp.
This is an extremely old and well-known problem with software interfaces. Either you version it or you don't. If you do version it, and change it, you change the version, and give people dependent on the old version some time to upgrade. Otherwise it breaks things, and that pisses people off. The alternative is not versioning it, which is a signal that there is no consistency to be expected. Any decent software developer should have known all this.
And while I'm at it: what's with the name flip-flopping? In 2014, GCP issued a PR release explaining It was no longer using "Preview", but "Alpha" and "Beta" (https://cloudplatform.googleblog.com/2014/10/new-release-pha...). But the link you showed earlier says "Alpha" and "Beta" are now deprecated. But no PR release? I guess that's our bad for not constantly reading the fine print and expecting it to revert back to something from 11 years ago.