Actually, I managed to reproduce the issue! I will do some further investigation and talk to the people who own the API and then get back to you 
In the meantime, it seems from my usage like this problem is only occurring for asApp, not asUser. Would it be possible for you to use asUser?
Update 8 March 2pm Sydney time:
@AlexFang It seems there are some complex permissions issues relating to copying attachments in comments, so it is not a trivial change to support this. I’ve chatted with the team responsible for the API and it’s something that they’re looking into but are unlikely to be able to get around to in the short term.
Their suggested workaround is to try parsing the comment body for attachments and add the attachments to the issue beforehand before adding the comment. Unfortunately, the add attachments API does not quite work in Forge yet, since it uses the multipart/form-data API, as we have discovered together already in Copying attachments Rest API Post.
While we work on the multipart/form-data functionality, is it possible in the meantime to parse the comment bodies for attachments and omit them from the copied comment?