I often do searches by tag name, manually surrounding the search term by square brackets. A search for [re:zero] finds no hits, though. In fact, the colon is auto-removed and the search is actually made for [rezero]. So that is actually what got no hits. Similarly, a search for rezero (no brackets and no inner punctuation) finds no hits, and it probably never will unless someone types it as part of their question or answer.
Note that if you search without the brackets, then re:zero does find appropriate questions. This is quite fortunate, because that is the actual English name of the series, with the colon. The results of that search showed me that the tag name is [re-zero], with a dash instead of a colon.
I suggest that the tag [rezero] be created as a synonym for [re-zero]. While it won't solve all search problems concerning the series, it would help some.