WordPress OStatus support

I build an OStatus plugin that wotks fine with Mastodon and GNU.social but I have problems to get it to work with pleroma. I hope you can help me.

Following works fine, but pleroma seems to not subscribe to the WebSub/PubSubHubbub - Atom-Feed. You can find an installed Instance here: pfefferle@notiz.blog (https://client.webfinger.net/lookup?resource=pfefferle%40notiz.blog). The code is on GitHub: https://github.com/pfefferle/wordpress-ostatus

The supported hubs are: https://pubsubhubbub.superfeedr.com and https://pubsubhubbub.appspot.com

Perhaps you can point me to the issue, because it is hard for me to debug pleroma.