We have an ice farm and there is a single block that does not form ice ever.  It is the same biome (mountains), same y-level (96) and the same light level (sky 15, block 0) as the adjoining blocks.  All blocks have full sky access.  The block in question is not on a chunk border.  Water is not flowing.

I don't see this reported anywhere.  It's a subtle thing, so may being missed.
I am not sure what I could provide to help troubleshoot this.