When you launch Scrivener, the search settings default to Search In = All, Options = Search Included Documents, Operator = RegEx.
If you change those (All to Text, Options add other choices) and then create a collection from the results, the next time you launch Scrivener, the collections contain documents they shouldn’t because the settings reverted to the defaults.
The search settings in place when you create a collection from a search should be part of the collection so each time you launch Scrivener, the collection returns the same documents. Otherwise, what’s the point of creating a collection from a search if every time you use it you have to re-adjust its search settings first? The collections become more problematic if you have some with conflicting settings; you can’t switch from one to another without first changing the settings every time.