There was no other prompt, no system prompt, etc. Many users have reproduced, exactly as it demonstrated in the parent.

Are you using the flash models? Reasoning models or extended thinking will change the result.

GPT 5.5. Instant shows the same error. If the given prompt isn't working, you can also try "300+140=460 is this correct?". I suspect that leading with the equation may be part of the issue, but haven't tested much.