Setup: * Set AutoCreateAuthorities to generate, AutoLinkBiblios to Do, CatalogModuleRelink to Do (and EnableAdvancedCatalogingEditor to enable if you want to make your life easier) * Ensure that LinkerConsiderThesaurus is at the default Don't * Make sure you don't have a genre authority record for Boring stories Add the following headings to a bib record and save it 655 _ _ ‡aBoring stories 655 _ 0 ‡aBoring stories 655 _ 4 ‡aBoring stories 655 _ 7 ‡aBoring stories‡2gsafd 655 _ 7 ‡aBoring stories‡2lcgft Note that you now have each heading linked to a different authid, and you have five genre authority records for Boring stories, all identical. If instead you already had a Boring stories authority record (no matter the thesaurus) before saving the record, they would have all been linked to it.
Seems like it must be from the linker caching with $thesaurus in the key, and only Heading knowing that LinkerConsiderThesaurus is Don't, but it's not obvious why.