Slack subscriptions will not send or show up in logs

Slack alerts only send if I click "Send to Slack now". No subscriptions have worked in the last 6 months of trying to set them up, whether it's to a private user or to a channel. No errors have ever appeared in the logs about it, and nothing regarding the subscriptions shows up in the logs at the time that they are set to run. What am I possibly missing here?

Deployment: via docker on an EC2 instance. The instance has been rebooted, the container has been re-created, etc. Still never worked

metabase.task.send-pulses.send-pulse.job indeed shows the 2 hourly subscriptions I set up, and it has the next fire time, but it just doesn't ever fire.

Diagnostic info:

{
  "browser-info": {
    "language": "en-GB",
    "platform": "MacIntel",
    "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/137.0.0.0 Safari/537.36",
    "vendor": "Google Inc."
  },
  "metabase-info": {
    "databases": [
      "postgres",
      "h2"
    ],
    "run-mode": "prod",
    "plan-alias": "",
    "version": {
      "date": "2025-03-18",
      "tag": "v0.53.7",
      "hash": "1cd6e9d"
    },
    "settings": {
      "report-timezone": null
    },
    "hosting-env": "unknown",
    "application-database": "postgres",
    "application-database-details": {
      "database": {
        "name": "PostgreSQL",
        "version": "16.8"
      },
      "jdbc-driver": {
        "name": "PostgreSQL JDBC Driver",
        "version": "42.7.4"
      }
    }
  },
  "system-info": {
    "file.encoding": "UTF-8",
    "java.runtime.name": "OpenJDK Runtime Environment",
    "java.runtime.version": "21.0.6+7-LTS",
    "java.vendor": "Eclipse Adoptium",
    "java.vendor.url": "https://adoptium.net/",
    "java.version": "21.0.6",
    "java.vm.name": "OpenJDK 64-Bit Server VM",
    "java.vm.version": "21.0.6+7-LTS",
    "os.name": "Linux",
    "os.version": "5.10.0-28-arm64",
    "user.language": "en",
    "user.timezone": "GMT"
  }
}