I've started noticing this issue before. When I first saw this happen, I just removed the URL from the address bar using the shortcut Shift + Del. Unfortunately, this didn't last long, but I just ignored it and lived with the characters disappearing, until it started annoying me a lot again. Looking for a solution, I found this question so I dug deeper since I now knew I'm not the only one.
What I have figured out so far is that those are search engines. Chrome has some magic going on where it detects search engines on websites you visit and adds them to your browser so you can utilize them by typing in (part of) the URL and pressing Tab, as you already know. This was my first clue where to look.
As I looked through my search engines, I found the offending Google entries, having either one or two underscore characters appended to their keywords. Because
source=hp was part of the query URLs, these must have been created when I went to the Google homepage and then searched for something from there. As I went looking for more of those entries, I also found two additional entries for reddit, which is where I started seeing a pattern.
I turned to the Chromium Code Search and tried figuring out how this happens. As it turns out, there's a unit test specifically for scenario, which made finding it a bit easier. How I understand it is that if a search engine exists and Chromium wants to add another search engine with the same keyword, it will try to generate a new keyword from the URL. If the generated keywords already exists as well, it will append an underscore to the original keyword and go with that. The code doing the actual work can be found here and here.
TL;DR: Unfortunately, none of this actually explains why those search engines get added, but it does explain that it is the intended behavior under some special circumstances and not some kind of extension going rogue. I also don't have an explanation for why some of the characters disappear from the search query and I'm not really sure where to start looking either.
For now, I manually removed the search engines from my settings (direct link is chrome://settings/searchEngines or right click on the address bar and click "Edit search engines...") and a few quick tests (going to the homepage and searching for something there) didn't make them reappear, so fingers crossed.