AIUI, the forum software is a standard open-source project - not a SiWi bespoke thing - so some things (including limitations) will be inherent in the standard project.
So I don’t know how much control SiWi has over these things…
Anyhow, there have been some known limitations with “special” characters in search strings - maybe the underscore is one of them…?
In the ‘Advanced search’, the ‘Search for all terms or use query as entered’ option should do what you want for “time_t unresolved” ?