Alerting on Slack using webhooks

Unfortunately it is only possible to configure Slack integration using old fashioned webhooks that can talk to single channel only rather than using proper url with token that can talk to multiple channels. Because of that to differentiate alerts I am using Email + Slack integration for one type of alerts and Email + Webhook for another. For some reason alerts using webhook are not being sent to Slack (I can still get an email). There are no errors and it is not possible to test it within the Atlas UI but I can confirm it is working as expected. Why is that?