After investigation I found that the issue is actually not dropping in but instead, as default autocomplete behavior is to look and expand WORD in 2 ways by default – prefix and suffix.
So what we see is simply in expanded to chains. To prevent it we can use 0 as prepend to make it expand words only in the suffix way: Searching > Autocomplete | Manticore Search Manual
Actually, I’m not sure what we can do here. Probably to improve the possibility of autocomplete we should think about contextual preservation in the whole phrase.
Autocomplete is handled by the Buddy, you can find the logic here: manticoresearch-buddy/src/Plugin/Autocomplete at main · manticoresoftware/manticoresearch-buddy · GitHub
This is still a new feature and we still think what we can do better with it, but in your case I think the good option to start is to try to use 0 as prepend option.