> So, I seem to be getting the driver code in a state where #isClose()
> returns false even after FBManagedConnection#destroy has been
> called.

Connection#isClosed() checks only if there's valid managed connection
associated with the connection. However, it seems that
FBManagedConnection#destroy() method (it sets db handle to null) is
invoked not as the result of Connection#close() call, but in some
other situation... Without a reproducable test case I hardly can say
anything right now...

Roman