I'm not sure why but everything returns true for Matches for just the search string ms as well. Am currently investigating but haven't touched LUA in a long time and I'm not even sure how to run the test suite.

Suspecting it was similar to the situations that you had tests for it does indeed look like Mythics have their own API:
https://www.townlong-yak.com/framexml/live/Blizzard_APIDocumentation#C_MythicPlus

turns out keystone and pet item links are broken. it should not be possible to search for both by name.