This is moreso a gameplay issue, not art

For clarity, I read this as a static map design issue instead of an engine issue, which roughly approximates to the art label.

My bad, sorry if I sounded rude

The gargoyles are collectibles that move around the pre-determined areas in the map.

This is weird. Community maps use ´tf_halloween_gift_spawn_location´ to locate where the Soul Gargoyles spawn. For example, in Ghoulpit:

However, Helltower doesn't have that entity in any spot, so I guess it's hardcoded?

Helltower doesn't have that entity in any spot, so I guess it's hardcoded?

I did a few checks, and it is.
Mann Manor, Eyeaduct, Ghost Fort, and Helltower all have their Soul Gargoyle locations hardcoded in the code.
Helltower has a spawn point near 10225 7462 -367 and another near 10209 7768 -367
(you can use the setpos command to go to these locations if the server has cheats enabled)

While you could fix this by just removing these 2 problem locations from the code, it would be way better to just scrap these hardcoded locations entirely, and add them to the map files.

I guess it's hardcoded?

Man I went digging for these locations to make maps for the wiki and it was awful if I remember right (did this in December last year). The conclusion I came to was some maps use the entities (which makes locations findable and even spawn outside of the halloween event in listen servers) and other maps rely on the hard coded values, and then some maps use a post-load list directly from the GC that we can't access. I believe I sourced that last bit of information from the alliedmodders community forums. (I also requested these locations last year via email, any chance on a followup :))

That said, having helltower's gargoyle location(s) change this year makes me think every location is handled by the GC's list, that location almost looks like it might be a mirror on 2 axis for the spot on the hill so maybe there was some copy paste error and some negative signs thrown in the mix?

Edit: forgot I left a comment about my findings on the wiki page too: link

Edit2: If my previous comments are accurate then this is possibly a lag related issue since the client is handling the rendering. Similar to how a large lag spike may render other players invisible, but perhaps only occurring if the lag spike occurs when the server sends the next gargoyle position? I've only seen this posted one other time on reddit (might be the same person) so it seems rare as I would assume this being a legit location would make it very common (and thus reposted to death). @hobophobe would you happen to remember if you had lag spikes (or symptoms of one such as aforementioned invis players) during this match?

That said, having helltower's gargoyle location(s) change this year makes me think every location is handled by the GC's list

The locations didn't change this year, you can play with old versions of the game (you can download old depot manifests with SteamDB), and the Gargoyles will still spawn here.
You can even go on a listen server on the current TF2 version and set host_timescale to 30 to speed up time, and you'll eventually see a Gargoyle spawn in this spot.

In halloween_gift_spawn_locations.cpp, under static valid_item_pos kValidPositions_Hightower, there are 2 invalid coordinates (10225.334961, 7462.086914, -366.968689 and 10208.725586, 7768.193848, -366.968689) where one spawns the gargoyle inside the rock right under the portal, and the second one spawns where the OP took the screenshot.