MrSteveVee Xigua nailed it, it's definitely Haze Removal.
I had deleted the photo as it wasn't any use and not a great shot due to being taken without a tripod at night in challenging lighting conditions, but I was able to restore it for testing.
I tried exporting as .jpg and even jpeg 2K and both had the same block colour issues. I tried exporting in PNG before and after disabling Haze Removal and the image was fine after disabling Haze Removal and extreme block colour before.
In case it helps with troubleshooting the actual issue with Haze, I always shoot raw, but this was in .jpg source as it was taken from phone.
Also, strangely it does show in Lighttable, but not in Darkroom.
I didn't see it in Lighttable when I first did it, but I might have been a little quick on the export and it takes a bit longer to actually render the defect.