Name for almost random memory distribution on the heap
This is fragmentation.  See https://en.wikipedia.org/wiki/Fragmentation_%28computing%29 for a definition.  Fragmentation can happy on any kind of storage; most commonly it is discussed for persistent storage (e.g., in filesystems), but heap fragmentation can also happen, and that is what you are describing.  See also https://en.wikipedia.org/wiki/Memory_management (which discusses heap fragmentation).