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I have not been able to find any instances of an itch.io staff member directly stating that search results are truncated (the only person I found who suggests this is you, oddly enough). What I did find was multiple instances of the staff clarifying that search results are optimized for searching for exact titles.

"The box is optimized for exact title searches, that's what. Searching by keywords is a matter of luck." (No Time To Play, Why is the search function of the store so limited?)
I've said this in other posts, but essentially search is suited for finding things by direct title, not by topic. Tags are how people browse things by topic. People do not browse by topic...through our search engine. Hope that helps. (leafo, My game doesn't show up in search or tags)
"Our search engine is optimized for Direct title search since that is how the majority of people use search. For users who are browsing for new games, we try to push them into our browse pages with tags, which are surfaced on the top of the search results. This means that the text in the body of your project page and any tags you add will not become search queries for your project. Tags are only for our browsing pages, which are separate from search." (Limitations of Search, Getting indexed on Search & Browse)

So no searching of description. Maybe tag searching will be added, if I understand leafos announcment correctly. The wording is vague.

But if search results are not capped, why could we both not find your monsters asset pack? I tried several of your packs, most I can find with the search button.

I will also not find the pack Slimes | Pixel Asset Pack by searching for slimes. I did get a result of 72 items. It can be assumed that there are at least 73 items with that word in their title. This is capping/truncating.

And the results for that monster thing was capped at a lower number. It is still capped at 61 if you activate adult items. 

The capping for tetris is 181. Of the 1600 games tagged tetris, most have tetris in their name. So I searched some of those games with tetris in their name along other words in the search 180 search results. Many do not appear there. And only 123 games are named exactly tetris in the search result, so there should be 60 games with tetris plus other words. Only 60.

Those 60 are what I assume to be a subset of matches that will rotate in some manner to expose every game once in a while to such a search. I am not confident, if it will always return all exact title matches, but I am confident, that it will not return all matches where multiple words are involved.