It's a good question: why are the GAAP rules what they are? It needs to be investigated, though.
The reason it happens in multiple quarters is likely that Anthropic's valuation has increased in multiple quarters. Every time Anthropic's valuation increases, the convertible bonds are worth more money. Other shareholders have "lost" money compared to what would have happened if Anthropic had sold regular bonds instead of convertible bonds. Except that no, the value of their shares is increasing too. An increased valuation is actually good for them.
That's how equity works. If someone owns 1% of a company and its valuation doubles, their share is worth twice as much and so is yours and we wouldn't even normally consider it to be diluting the other shareholders.
So the question is whether convertible bonds are worth more than regular bonds and the answer is, yes, absolutely, but they are partially equity, and it's the equity part that's increasing when the company's valuation is increasing.
And if the company's valuation went down then the value of the equity part of the convertible bonds would decrease. Maybe that would be a profit by GAAP rules?