If you set a custom search filter and delete one of the resulting passwords, the password list resets to your full list even though the search filter stays at the top.
I searched for "All Items" with "Date Modified" "is less than" "3/13/2012". When I deleted some of the passwords (cleaning out old items), the password list reset as though I did not have a custom search. The custom search was still set at the top of the window. Changing one of the date values does re-display the search results properly.
1Password Version: 7.2.5
Extension Version: Not Provided
OS Version: 10.14.2
Sync Type: 1Password Account