I make this point in most of these threads, but this is falling to an anecdata fallacy. Gmail is thousands or tens of thousands of times bigger than Fastmail[1]. When you have have a scale difference that large, you're going to have a huge, huge imbalance of problem reports like this that has nothing to do with actual reliability. All providers mess stuff up occasionally. Everyone has a customer support crisis at some point, with someone.

In point of fact I'd put the odds of Fastmail failing entirely as a business rather higher that those of any single user having an unresolvable 2FA glitch with a gmail account. In the world of real data and not anecdata, Big Tech is incredibly reliable.

[1] Which seems great, btw. I'm actually looking at moving my vanity domain to them as I'm sick of chasing standards trying to host it myself. This is absolutely not a ding at Fastmail.