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I downgraded to stable and the issue went away. It's not a huge deal, and it could just be a quirk with our instance.

Groups are built like this:
Session filter: CustomProperty1 = 'ABC'
Subgroup Expressions: IIF (GuestOperatingSystemName LIKE '*Server*', 'Servers', 'PCs') + ' - ' + CustomProperty1, GuestOperatingSystemName

Any search at all in a subgroup finds no results.

I assume it was a bug with the pre-release as I've never ran into this before, support advised I post here, but perhaps it is specific to our instance..

Example - PC 'test' is in root group 'ABC' and in sub-group 'ABC-PCs'. So it is visible from both groups. Searching 'test' in 'ABC' finds the PC as expected, searching 'test' in 'ABC-PCs' shows no results (even though PC is clearly visible in the subgroup)