I have finally got my brain around this.
There was definitely something quirky about this, but
=> basically, the issue was that in one of the two months in question, there were no records matching all of my WHERE_clauses - so nothing was loaded in that iteration of the loop.
=> I fixed this now by building a "failsafe" - a dummy_record to be loaded when that happens so that I'll have something to work on.
Consider this done.