I am experiencing the same problem and it seems to be a known integration issue (from Slack's point of view).
As described on slack.com:
This integration only supports the Repository trigger. time, so make sure it is selected. Click Save when done.
I'm not sure where to find a public link to this document, but you can find it if you want to add bitpack integration. And the slack documentation is apparently being updated with new bitpack web sites.
I hope that one of the parties will fix this soon.
Update: It seems that integration has started working in the last couple of days. At least for us with PR create / update / comment? Approve / merge.
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