This specific issue may fall within the bounds of this larger bug: Bug 11270 - Search Result highlighting should be refactored https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=11270 Search terms are not highlighted in search results if the terms are truncated. For example, the search kw:mold will show results with every instance of "mold" highlighted, even if it appears within a larger string like "molding." But a search for kw:mold* or kw,rtrn:mold will not show any highlighting in the search results.
Still valid in 21.05
It seems to break when using any 'qualifier' after the search index, like ',phr' or ',rtrn'.