I finally found a soul-crushing short story that had been stuck in my head since 2016: Prodigal by Gord Sellar. The issue of Analog it first appeared in is no longer in print but it’s available through the Wayback Machine here.
I don’t want to spoil it. I’ll add a review when I add some support for spoiler tags to this blog! It’s a 15-page story about a dog owned by a couple that receives him a treatment that grants him the intelligence of a human child and the ability to speak.