It is my understanding that there is no such thing as a strict rule
as to how that is to be sung.
Agreed. I'm English (through no fault of my own) and in forty+ years of singing I don't recall coming across a formal statement about this. The practice might have arisen from the frustration of composers confronted with bad amateur choirs whose singers were careless about the placement of final consonants. We've all heard the results: half a bar of sibilance when the word ends in "s" for example. Conductors will often instruct choirs precisely where to place certain word-endings, having regard to ensemble, harmony, acoustics, etc., and often consciously amending the written score.