Cosmog no longer appears in the possible legendary spawns list in Sunflower Plains at dusk, dawn, or any other time. Celebi showed up just fine, as did Shaymin in the flower forest next door.
I went to check the wiki to make sure it was the right area for Cosmog, and the Legendary Spawns page claims it is while other pages like Sunflower Plains doesn't list Cosmog at all. It isn't listed under the Ultra Space biomes either.

I can still make it appear via /pokespawn , but it just doesn't show up on its own. Was this a possible oversight in the update or wiki info or what?

Thanks for the report.

Do you have external spawning JSONs enabled, by any chance? And if so, when were they generated? Are you looking at the correct times from our Spawn Time page? Is your biome large enough so that any potential spawns are outside of the no-spawn radius around the player? (minimumDistanceFromCentre)

I can confirm it spawns in a Sunflower Plains biome during Dawn/Dusk (both from looking at the spawning file and from personal experience), though it should also be showing up in /checkspawns legendary.

I didn't manually set any up as far as I know. I'm not sure where to find the original spawning file, never mind any custom(?) spawn files. Could you tell me where that file is?

If it helps, what I did to set this world up was install Pixelmon 7.0.7 as a completely different instance, ported over the mods, luckperm settings, Wonder Trade settings, and then manually changed the settings in pixelmon.hocon so that I would avoid overwriting the new code from the update.

Yes, I am certain about when I checked the large Sunflower Plains I was in as the previous world had one spawn in front of my house near-daily. I even paused time just to fly around at dawn and dusk to check various spots in the area. Dawn returned with Celebi, and dusk didn't list anything.

The original files are in the .jar, and they get moved over to an external folder if that setting is enabled. If you haven't enabled those before (you would have a "pixelmon" folder in the server root if you did), things should be fine there.

Are you using any world generation mods? Not too sure what else this could be -- I just confirmed again that I'm getting spawns on 7.0.7, on a fresh world.

Our Discord may be worth joining here, also. They may be able to drill deeper into this.

As I said, I only ported over the mods from the last server and didn't notice anything strange the last time. Minecraft is strange that way I suppose.

I did find the folder however and I'm guessing Biomes o' Plenty might've had something to do with it? The world gen should just be vanilla, but Cosmog appears to be listed for some biome called "mutated_plains".

Would it be safe for me to just edit it to "sunflower_plains" or should I instead try to nick the files from a fresh install without any mods on it?

(Apologies, can't find an edit button.)
Just realized BoP is a required mod.

I went and checked other "mutated" (and oddly complex/specific) biome legendaries (Kyurem, Landrous, Jirachi), and they all appear in the /checkspawns legendary list just fine. It seems to only be Cosmog having this issue.

"mutated_plains" is 1.12.2's internal representation for Sunflower Plains, that's all good. Try removing the "pixelmon" folder after backing it up, and then restarting the server -- that should create a fresh set if any externals are enabled.

The only "pixelmon" folder I see anywhere in my files is within the Pixelmon jar, and I'm pretty sure you don't mean that one.
This issue is also affecting my singleplayer, so it's most likely just another mod causing it. I just don't know which one or how it could possibly be interrupting only Cosmog and no other spawns (that I'm aware of). I could either spend several hours going through each one at a time, or I can save myself a headache and just do a mini event for it.

I appreciate you trying to help me all the same, and I apologize for this mess.
If you want, I can compile a list of my mods for you, or I can pack up my instance and send it to Google Drive if anybody's just interested to look at it. If not, that's okay, my server group is pretty small so doing a /pokespawn workaround isn't a big deal.

Ahh, I thought you'd found one in the server's root from your earlier post. Can you send me that mod list on Discord? May be worth throwing it into #server-support as well, so other people can have a look too.