Posts from Friendica via AP aren't accepted - only replies

Friendica is currently implementing AP. Tests with all systems are looking fine - except Pleroma.

Replies to posts from Pleroma are accepted, but posts that aren't replies don't appear at Pleroma.

This is how a post looks like:

{"@context":["https:\/\/www.w3.org\/ns\/activitystreams","https:\/\/w3id.org\/security\/v1",{"ostatus":"http:\/\/ostatus.org#","uuid":"http:\/\/schema.org\/identifier","sensitive":"as:sensitive","Hashtag":"as:Hashtag","atomUri":"ostatus:atomUri","conversation":"ostatus:conversation","inReplyToAtomUri":"ostatus:inReplyToAtomUri"}],"id":"https:\/\/pirati.ca\/object\/ec054ce7615ba9d7162977d159354203","type":"Note","summary":null,"inReplyTo":null,"uuid":"ec054ce7615ba9d7162977d159354203","published":"2018-09-25T06:35:02Z","url":"https:\/\/pirati.ca\/display\/ec054ce7615ba9d7162977d159354203","attributedTo":"https:\/\/pirati.ca\/profile\/heluecht","actor":"https:\/\/pirati.ca\/profile\/heluecht","sensitive":false,"context":"https:\/\/pirati.ca\/object\/ec054ce7615ba9d7162977d159354203","conversation":"https:\/\/pirati.ca\/object\/ec054ce7615ba9d7162977d159354203","content":"@<span class=\"vcard\"><a href=\"https:\/\/pleroma.soykaf.com\/users\/heluecht\" class=\"url\" title=\"Michael (Pleroma)\"><span class=\"fn nickname mention\">Michael (Pleroma)<\/span><\/a><\/span> Test 2","source":{"content":"@[url=https:\/\/pleroma.soykaf.com\/users\/heluecht]Michael (Pleroma)[\/url] Test 2","mediaType":"text\/bbcode"},"attachment":[],"tag":[{"type":"Mention","href":"https:\/\/pleroma.soykaf.com\/users\/heluecht","name":"@heluecht@pleroma.soykaf.com"}],"to":["https:\/\/pleroma.soykaf.com\/users\/heluecht"],"cc":[]}

And this is how a reply looks like:

{"@context":["https:\/\/www.w3.org\/ns\/activitystreams","https:\/\/w3id.org\/security\/v1",{"ostatus":"http:\/\/ostatus.org#","uuid":"http:\/\/schema.org\/identifier","sensitive":"as:sensitive","Hashtag":"as:Hashtag","atomUri":"ostatus:atomUri","conversation":"ostatus:conversation","inReplyToAtomUri":"ostatus:inReplyToAtomUri"}],"id":"https:\/\/pirati.ca\/object\/ec054ce7175ba9d6d26d72d764066009","type":"Note","summary":null,"inReplyTo":"https:\/\/pleroma.soykaf.com\/objects\/082ca304-114f-4601-bef7-898ce6dfef99","uuid":"ec054ce7175ba9d6d26d72d764066009","published":"2018-09-25T06:33:54Z","url":"https:\/\/pirati.ca\/display\/ec054ce7175ba9d6d26d72d764066009","attributedTo":"https:\/\/pirati.ca\/profile\/heluecht","actor":"https:\/\/pirati.ca\/profile\/heluecht","sensitive":false,"context":"https:\/\/pleroma.soykaf.com\/contexts\/0bff6153-8239-4540-8544-3d52015409ce","conversation":"https:\/\/pleroma.soykaf.com\/contexts\/0bff6153-8239-4540-8544-3d52015409ce","content":"@<span class=\"vcard\"><a href=\"https:\/\/pleroma.soykaf.com\/users\/heluecht\" class=\"url\" title=\"Michael (Pleroma)\"><span class=\"fn nickname mention\">Michael (Pleroma)<\/span><\/a><\/span> Antwort","source":{"content":"@[url=https:\/\/pleroma.soykaf.com\/users\/heluecht]Michael (Pleroma)[\/url] Antwort","mediaType":"text\/bbcode"},"attachment":[],"tag":[{"type":"Mention","href":"https:\/\/pleroma.soykaf.com\/users\/heluecht","name":"@heluecht@pleroma.soykaf.com"}],"to":["https:\/\/pleroma.soykaf.com\/users\/heluecht"],"cc":["https:\/\/pleroma.soykaf.com\/users\/heluecht"]}

Where is the difference that the one is displayed, the other one not?