This is implausible; you're assuming that every civilizational stage from every point in this hypothetical species development would have had the same burial practices?

Though, I'll grant it's possible they could've been soft-bodied creatures that leave very little in the fossil record or might otherwise look ambiguous to us. A future civilization looking back on human fossils might think we lived no differently than any other Hominidae. Anatomically modern humans have been around for ~2.5 million years, if our civilization were to end in even another couple million years that would still be a fairly tiny window in which we developed the intelligence to create civilization and then went extinct. If some jellyfish species developed human-level intelligence 300 million years ago and then went extinct in the same window, it's entirely possible we wouldn't be able to recognize the difference between their fossils and those of their less intelligent ancestors/descendants.