I've had that happen many times with comments from you. I suspect it happens if it gets enough upvotes to send a notification and appear, but then gets a downvote which makes it get hidden again. 
It's rather interesting that I've seen it with your comments dozens of times before (suggesting a rather large number of people think you aren't being civil) while this is the first time you've ever seen it with me.
More interestingly the comment in question had nothing particularly controversial to it - I was merely suggesting you read up on some pertinent history and consider how this may affect your perspective on this era. 

The biggest flaw in the 'judge them by their time' argument in this case is that what Lee stood for was considered evil by the vast majority of people even in his own time. So much so that people an ocean away were prepared to starve to help end it.
https://en.wikipedia.org/wiki/Lancashire_Cotton_Famine