Maybe it indeed is. Or do you have a proof it is not? If you use a watch with an oximeter, it probably knows relatively well whether you are sufficiently oxygenated or not, hence whether you work in the aerobic mode or not. If you have an older generation of a watch, then it likely only estimates the aerobic/anaerobic mode from the HR data. In that case you should consider reviewing your HR zone settings, and adjusting them accordingly, if the results do not match your expectations.
OK, sorry, I've read only the body of your post, and did not notice it stated the contrary of the title. Please post screenshots demonstrating the problem.
