Blocker Description
An array with a trailing optional element followed by a scalar optional element loses a separator when both elements are missing during an unparse. The trace indicates, there's a SuppressableSeparatorUnparserSuspendableOperation for the scalar element (se5 in the example schema). My initial guess was that the last element in the array was being treated as a trailing element, but we need it to not be treated that way.
For completeness, you get the following error from TDML on the 2nd test
output data length 12 for '/1/2/3.5//6 ' doesn't match expected length 13 for '/1/2/3.5///6 '