Memory-Optimized Table Memory Allocated issues
You should check out this blog post from the king of In-Memory OLTP, Ned Otter:

Row version lifecycle for In-Memory OLTP

In particular, this passage:


  2. Deallocating rows from memory
  
  ...
  
  So the two triggers for Garbage Collection are memory pressure and/or transactional activity. Conversely, that means if there’s no memory pressure – or transactional activity is low – it’s perfectly reasonable to have row versions that aren’t garbage collected. There’s also no way to force garbage collection to occur.


So this is just to clarify that there are situations where garbage collection may not occur, or may occur slowly.

The post goes on to discuss that long-running queries on in-memory tables will completely block garbage collection, so make sure to check for any of those.



The reason the screenshot of SSMS is off (used it higher than allocated) is because it's purely based on the DMVs, so the NULL memory_allocated_for_table_kb throws off the calculation.  This is what runs when you open the database properties window:

SELECT
    isnull((
        select convert(decimal(18,2),(sum(tms.memory_allocated_for_table_kb) + sum(tms.memory_allocated_for_indexes_kb)))
        from [sys].[dm_db_xtp_table_memory_stats] tms)
    , 0.00) AS [MemoryAllocatedToMemoryOptimizedObjectsInKB]

SELECT
    isnull((
        select convert(decimal(18,2),(sum(tms.memory_used_by_table_kb) + sum(tms.memory_used_by_indexes_kb)))
        from [sys].[dm_db_xtp_table_memory_stats] tms)
    , 0.00) AS [MemoryUsedByMemoryOptimizedObjectsInKB]




I'm still not really sure what the purpose of that object_id = 0 row is in dm_db_xtp_table_memory_stats.  I can repeat the behavior locally (create a table with LOB data, then ALTER the column to fit in-row, suddenly I have that same 0 result), but I don't know if it's a problem.  

It's possible that it's normal overhead related to in-memory OLTP, but it might be a bug.  If it's causing significant problems, I'd suggest opening a case with Microsoft.  There have been bugs before related to memory usage with LOB data, see MSDN Forums | In Memory OLTP for an example, and this CU: FIX: Out-of-memory error when you run a query to access LOB columns through In-Memory OLTP in SQL Server 2016.