As I mentioned in my post from what I can test it's because tags used on a number below 200-300 games will not be autocompleted.
That has nothing to do with this. The list is arbitrary. It is neither the number of games having that tag, nor if the tag has a description. It is just a list that has tags in it and is managed by itch in whatever fashion they deem usefull.
There is of course a correlation. Popular tags tend to have a description and be on the list. The new autocompletion feature is built regularly with a digest of the popular games. I do not know how they include the tags in there, but I would assume it is the same mechanism that would give a tag as a search result.
You cannot search for any tags. To "find" the tag boxing, you have to type it in verbatim in the tag box.
Based on your finding with the short description, I assume the search might populate the results with titles that have relevance in the short description, if there are not enough other results. Or they would appear but are truncated due to not having title matches, and too little relevance. I would assume with fishing that there would be titles that have fishin in description, but not in title. But there are so many exact title matches and enough partial title matches, that description matches are drowned out.