A correction is in order (as embarrassing as it s after Likes)
It should teach me to more thoroughly test before posting results.
Anyway, I decided to repro and had LikerGal1 Like the same parent Cat OPs .
No repro, 
Being a comparatively small localhost forum a lot of numbers have not diverged greatly and I looked for other “two occurrences events” that might be involved.
LikerGuy1 had also Liked 2 “hidden by Flags” posts.
LikerGal1 did similar, - No repro,
LikerGuy1 had also Liked 2 posts made by anonymous-mode members that were subsequently deleted by a Mod
LikerGal1 did the same - repro !!
Was it because the posts had been made in anonymous-mode?
A friendly member made a test post. Another Liked it, then it was Mod deleted. - repro !!
So unfortunately this isn’t as edge case as a topic-less category getting a Like.
It seems that Liking any post that is subsequently deleted will decrement the post like count, but will not decrement the associated topics like count.
Still a bit edge case I suppose, but maybe worth looking into if topic like count is used in value weighting algorithms.