Thanks to everyone who replied. Here's a brief summary of those responses:

This is an old problem. It has been reported on several occasions: over the last decade, three of those reports occurred in months beginning with the letter "A".

A little over fifteen years ago, tye reported in "(tye)Re: Node reply count bug?":

"There is currently a table to track number of replies. It is being phased out ... When that work is finished, such problems should disappear."

Maybe that refers to an unrelated Reply Count problem; however, if it doesn't, can anyone advise how that work is progressing?

I had a look through Tidings: I couldn't find anything relevant. I then looked through (historical) Tidings: "Number of Replies clarification Oct 11, 2008 at 03:00 AEDT" (looks like you should adjust for your timezone) was the only related entry I found. Adjusting my User Settings, as that describes, to just show direct replies, tells me that Nodes to Consider post (from my OP) has one reply instead of three: sadly, it still has zero replies (as I originally reported).