Unfortunately, the module supposes both node and comment body properties are always default ones. For various reasons often time users delete $node-body or $comment->body and use custom fields instead. Would be very nice, if the module had admin page where users could indicate what are node and comment body fields and then module could use the user-set fields.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I'm in two minds about this. I agree, it's probably even more likely that a D8+ site won't be using the default body field. On the other hand, this could turn into a rather complex field mapping exercise, which could be avoided in most cases by using the same field name on entity and comment.
I will put on the list of features to consider, and welcome further comments.