Has this ever been solved?
Issue is that the above sample only works for posting and in plain text, But chatterposts are RichText formatted and soon you try to update the FeedElement you lose the formatting ( like in Richtext to Longtext ) you get all the <b> </b> in the updtaed post

here is the sample that adds the @mention but lost the rich text formatting as from my understanding it creates first a complete new Item to then override the original item...

Actual problem is that if a user forgets the @mention the question is never been seen, so we aim to add the @ via apex

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);
                }
            
            }
        }
    
    }


}