I know Tapatalk is a third party app that you guys have nothing to do with, but, if you know anyone on the tapatalk team you should let them know about this bug :

When an original post has a title longer than 85 chars, and you reply to it in tapatalk, you get an error "title is too long - please reduce the title to 85 chars".

However the title is not editable when replying. So I think tapatalk is defaulting to the original post title in the reply, then some tapatalk imposed character limit kicks in and prevents you from submitting. It might also be an mtbr configuration issue : if the character limit imposed over Web is longer than the character limit imposed over API, then this could happen.