I don't know if this is a general issue but I have seen it in ealier Safari versions as well for years now when doing Google searching. To explain I will give an example. If I do a Google search, then after seeing the result change my search parameters, click on a result then attempt to go back to the previous search, I am taken back to the original search. It appears as if the history trace of the redefined search is gone and I have to manually retype it in.
Like so in order:
GSearch 1 -> GSearch 2 -> Link
When going back one GSearch 2 is missing or somehow removed.
I tend to start in the Safari search bat then use the search widget on site. But I haven't noticed it all the time. Just when a link has gone missing. :-)
I know that behavior - but I haven't investigated yet whether it's a bug or intended like this.
Intended to be annoying then. ;-)