This is really more about archaeology, biology, and/or chemistry than physics. Of course mass-energy is conserved. That's not the issue.
Compacted lava ash is porous. Liquids and maybe even microbes can squeeze through it.
Except for bones, everything in a human body is biodegradable and/or liquid. And bones can dissolve too, given enough time.
In other words, the body shaped spaces in Pompeii were not mysterious voids of vanished mass. They were filled with leftover gases, some bone fragments and maybe dessicated tissue, after the rest of the victim's decomposed mass drained and evaporated away.
Ok, that's probably close to the right answer. I'm not a Pompeii expert. An archaeologist or forensic MD would be able to explain it better.
In general, when any topic doesn't make sense, a curious person should ask "what knowledge am I missing about this subject area?" many times before considering "what if the laws of physics are wrong?"