This situation is addressed here: https://forum.inaturalist.org/t/can-i-only-disagree-with-annotation-or-can-i-also-correct-it/24238
and is part of the design of annotations (so not a bug, even if it can be frustrating).
Users can now delete annotations added to their own observations (see https://forum.inaturalist.org/t/inaturalist-updates-for-december-2023/47962 for this recent change). So you can ask the observer to delete and then enter the correct annotation value.