This was really weird. I posted on 9 May at 15:48 on the last page of a thread and bookmarked my post. When I went back to the thread I had posted on, I landed on the first page of the thread, which I thought odd.
I went to the last page of the thread and scrolled looking for my bookmark, to read from where I had left off, but could not find either my post or a deletion notice indicating that it had been removed.
So I went to my Mumsnet Bookmarks and found that there was an "empty" Bookmark, ie. the Thread Title and my Username but nothing to click on to jump to the post.
I had also Bookmarked the post in my own non-Mumsnet Bookmarks so I had access to the URL for the post. So I tried using that Bookmark, thinking that would do the trick. However, that also took me to the first page of the thread, not the last page.
This is where the post should be but there is nothing there and clicking on this link will take you to the start of the Topic thread:
https://www.mumsnet.com/talk/am_i_being_unreasonable/5526935-council-elections?page=40&reply=152200362
As it was a long post I had composed it off-line and I still had the text so I posted it again.
This time my post did not disappear:
www.mumsnet.com/talk/am_i_being_unreasonable/5526935-council-elections?page=40&reply=152205451
If this is a bug, I have not come across it before and am just glad that I composed my post offline so that I could repost.
ps. Moving the box for the Title of a new thread to a space after the Message Box is really weird. It took me ages to work out what was going on. It is very counter-intuitive to have the "Subject Line" after the Message Box.