Memory Profiler Incorrectly Reporting Data as Free

**Version of emscripten/emsdk:**
3.1.54

It seems like after a certain point, the memory profiler begins to report allocated memory as "free," despite not actually showing that data as free in the numbered totals. It's possible this might not be a bug. In general, I think better explanations of what's going on with memory here might be needed.

You can see this in action by using this [small app I built](https://wasm-memory-pressure.netlify.app/) ([repo](https://github.com/milky2028/wasm-memory-pressure-test)) to allocate specified chunks of memory in wasm in megabytes.