There’s an ingredient that creates suds in toothpaste called sodium lauryl sulfate, or SLS for short. It’s also in other stuff, like soap and shaving cream. And one of the side effects when it hits your tongue is it temporarily kills your ability to detect sweet things.
It turns out orange juice is pretty bitter if you remove all the sugar. And since the SLS prevents you from TASTING that sugar, you just taste more of the bitter stuff.
Just keep in mind, you’re supposed to wait 30 minutes to brush if you’ve had citrus, or it can damage your enamel.