Actually I would argue that if your weight isn’t being supported by your saddle, then you are in fact “standing”. Whether you are vertical or not, the weight is being supported by the pedals. It’s not surprising that they consider that standing. Even if Garmin restricts their metric to vertical standing, that’s not necessarily the only correct answer.
Left / right power accuracy is very easy to test and already validated that Assiomas are very accurate. That and cadence are the only metrics that really matter. You can get caught up in the minutiae of the cycling dynamics but they’re of very limited value, none more so than seated vs standing time. What value are you going to gain from having that data? How will it change your racing or training?
If it really bothers you, contact Favero support.