Thank you, Leo. I will confirm whether Acrobat 2020's iFilter is 32-bit; DOpus version is 12.30 x64. I had Reset the search prior to performing a new one, but will check again if there are criteria I may have missed.
So you're saying it's possible there's a scenario in which Quick Search (and Windows Explorer) would load a 32-bit iFilter and return results, but Containing text: won't return results because it's looking for an x64 iFilter? I had the understanding from this comment from Jon that Opus's internal Find command doesn't use Windows Search, and I thought that explained why we were seeing different results between Quick Search and Containing text.