Is it normal my comments keep disappearing on peoples posts?

It happens quite often to me, too. I'll put the effort into a comment that I guess the OP doesn't like and poof; all that effort wasted.
Those that do this are just cowards, who can't deal with a response to their post that might not agree with their fictional view of the world, so the run and hide under the bed, so to speak.
Oh well, I guess these are future book burners, practicing.

It means your comment offended too many people or the OP.

Maybe it got posted elsewhere. Mine will often wind up on a completely different post. I usually just reload the page to see if it was posted. If it wasn't, I go to My Stuff and click on My Comments to see where the comment somehow wound up. Then I delete it and repost it on the right post.

Or it could be that the OPs are hiding your comments for whatever reasons they have.

Yep, there are a lot of cranky ass delicate people on here

Ya sometimes that happens when I click on the post then it says URL not found