BUG: metadata substitution varies in behavior

Today I compiled to .doc format. I normally use a number of custom metadata fields, and I print them.
However, this particular project had several pieces of the metadata missing, and the compiler behaved differently for some than for others. The field in question was blank, both times.

In one instance, it included the blank metadata (essentially, printed an empty line); in another instance, it printed the custom metadata reference (<$custom:Field1>). Same field, both times. Same blank, both times.