Oops, I snubbed a key part of your question - and it was in the thread title too! This happens when loading the game. I'm afraid this is not a detail.
Does the bug occurs if you load the game immediately before the choice?
My new guess is that if you save the game in a position where a choice happens, but the choice is actually overriden by the fallback option, when you load it it is positioned at the brink between the sentence before the choice and the default sentence after the choice. Usually, this non-choice is just skipped and we fast forward to the next sentence, but by loading it in that exact point the engine has to display something and the blank sentence is delivered.
It looks more like a quirk of the Ink engine but I'm not sure.