When you use memory-optimized table variables outside the natively compiled stored procedures, a LCK_M_SCH_M (schema modify lock) is taken on the corresponding memory-optimized table type during destructing the table variable. In this situation, the concurrent declaration of memory-optimized table variables outside natively compiled stored procedures is blocked.
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.