> But if your setting or plot relies on something we know to be scientifically untrue, and you don't put some effort into explaining why it somehow works in your setting, it's fantasy and not speculative.

Agreed but only because "some effort" could be as little as a single paragraph.

> But detective stories don't work if your world isn't grounded in something real. Otherwise the reader can't reasonably build their own theory or deduce the answer because it's based on what the author thought was cool and not what logically connects.

If you only change a tiny bit, this isn't a major issue.

> The appeal of something like The Expanse just falls apart if you introduce a FTL engine just because it makes for a more dramatic story moment somewhere in the plot unless there is some serious justification as to why the author didn't just break all the rules of their world (which is supposed to be our world, but in the future).

That specific story falls apart but you could have lots of thematically similar stories with FTL. No need for "serious justification" unless you're trying to pull it out of nowhere halfway through the plot. If it's there from the start, there's no problem.