I know I am playing a thread necromancer right here but I was researching this myself and just ended up reading the source code for ThreadLocal here, on mscorlib.

In the source code we can see that ThreadLocal is using a [ThreadStatic] attribute on ts_slotArray field. While this attribute has no implementation, the compiler upon encountering it on some field creates a thread specific storage slot for that variable and thus even if your ThreadLocal is static, it doesn't matter as compiler will disregard this and generate code that still creates a thread specific storage slot for that value.

If you want to keep your values reusable, you could use a ConcurrentBag/Dictionary or create your own thread safe collection.