Hi James,
is there a way to add an @mention without loosing the formatting, I foolowed all samples and played around a bit but if  a user ask a question and does not at mention anyone the question is not seen by the responsible group. We tried Flow / Process builder but the following apex was the best shot so far except that we lost the Richtext formatting and i was not able yet to set anywhere isRichtext to true...

public class addAddMention{


    public void checkItems(List<FeedItem> fi){
       String name='Salesforce Superusers';
       List<CollaborationGroup> cg=[select id from CollaborationGroup where Name=:name LIMIT 1];
       String groupId = cg[0].Id;
       String communityId = Network.getNetworkId();
       List<FeedItem> tochange= new List<FeedItem>(); 
        for(FeedItem f :fi){
            if(f.Type == 'QuestionPost'){
                if(!f.Body.contains('@')){
                    tochange.add(f);
                }
            }
        }
        if(!tochange.isEmpty()){
            for(FeedItem f : tochange){
                String feedElementId = f.Id;
                ConnectApi.FeedEntityIsEditable isEditable = ConnectApi.ChatterFeeds.isFeedElementEditableByMe(communityId, feedElementId);

                if (isEditable.isEditableByMe == true){

                    ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
                    ConnectApi.FeedElementCapabilitiesInput feedElementCapabilitiesInput = new ConnectApi.FeedElementCapabilitiesInput();
                    ConnectApi.QuestionAndAnswersCapabilityInput questionAndAnswersCapabilityInput = new ConnectApi.QuestionAndAnswersCapabilityInput();
                    ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
                    ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
                    ConnectApi.MentionSegmentInput mentionSegmentInput = new ConnectApi.MentionSegmentInput();
                    mentionSegmentInput.id = GroupId;
                    messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
                    textSegmentInput.text = f.body;
                    messageBodyInput.messageSegments.add(textSegmentInput);
                    messageBodyInput.messageSegments.add(mentionSegmentInput);
                    feedItemInput.body = messageBodyInput;
                    feedItemInput.capabilities = feedElementCapabilitiesInput;
                    feedElementCapabilitiesInput.questionAndAnswers = questionAndAnswersCapabilityInput;
                    questionAndAnswersCapabilityInput.questionTitle = f.title;
                    ConnectApi.FeedElement editedFeedElement = ConnectApi.ChatterFeeds.updateFeedElement(communityId, feedElementId, feedItemInput);
                }
            
            }
        }
    
    }


}

A trigger is fired on the post and calls the checkItems method, if the type of feedElement is of question and if there is no @ in the body 
the ConnectApi code adds the @Mention ( in our case a Group called Salesforce Superusers )

issue is the out put looks like 

but it should be like

Hello Subject
done or not


Any ideas?

As far as i understand the whole documentation, we can't update the feedItem but replace it, but how to maintain the former body