I was not able to find a way to reproduce your issue, but rather than continue in comments on another answer, I'll stake a proper answer so we can edit it into shape.
- Google puts one entry into my history for each "search" and then press enter (two searches produce two history entries)
- Duck Duck Go puts two entries (two searches produce four history entries)
You may be on to something where the "second" entry is being suppressed on your specific profile or there's a setting to suppress it. If my theory is valid - you might only get one entry for Duck Duck Go and zero for google.
To reproduce my results - clear all search history and then toggle off all the items except Include search engine suggestions (although I don't see any of these settings below changing my results)
Here is what google search for two phrases shows in my history - first using Google - then using Duck Duck Go
I wonder if people that lack these history events have extensions or settings that may be affecting this. A quick way to isolate that would be to make a new user on your Mac and repeat the quick test above.