Not sure how this should be changed in discourse, but those 2 numbers are very different and 155 is misleading since all the replies were deleted. Would it make sense to not show the “number of deleted replies” when counting the “number of unread replies” ?
We have had this reported a few times before, it is fiendishly hard to fix.
If a large number of posts are deleted then unread count is artificially incredibly high. We could I guess do a client fix that caps numbers but it just replaces imperfect with more imperfect.
Problem with counting deleted replies is that there is no cheap way of do it. You may have read 17 of the 32 deleted replies.
lots of posts are auto deleted via general forum maintenance here (ie: “all replies to this topic will be deleted after x time”) , since Meta has been around since the beginning of Discourse.