It could be argued that TUC saw Praxis "explode"... but that it had already been in a state of destruction for many, many years before it finally went nova. It isn't like anyone from the Federation was going near the Klingon homeworld before TUC so the extent of the damage on Praxis may not have been known to Starfleet until it actually "came down" in TUC.
The ability of the Federation to observe from long range was clear, though. The Federation may not have known all that was going on, but right up until it exploded, pretty clearly long range scans indicated Praxis had a basically spherical shape. Otherwise, the computer overlay would have looked different.