I’m not sure whether this is a “bug” but I don’t think it can be an intended behavior.
Suppose that someone has annotated a plant as “Flowering” and you disagree, but want to say that it is “Fruiting”. You can both click “Disagree” on Flowering and also add the Fruiting annotation. But when someone adds “No Evidence of Flowering,” that disables all other options; and disagreeing does not re-enable them, so you can’t annotate what you do think is going on instead.