Entity-specific searches (like Cases, Accounts, or Custom Objects) in Salesforce return a maximum of 250 results. Lookup Filters applied to those search results only apply to the top 200 results. As a result, if the result you're looking for isn't contained within those top 200 results, you won't see it in the Filtered Lookup list.
In general, this means that many "expected" results for unfiltered, general searches (which would return more than 250 results) may not appear when the search is conducted with a Lookup Filter applied.
Notes:
The following workarounds help resolve issues where expected results are missing from Salesforce lookup searches filtered by Lookup Filters.
As a workaround, either remove the Lookup Filters, make the Lookup Filters less restrictive, or use more restrictive search terms.
There are some variations in how Lookup Dialog Search operates, but performing a blank Lookup search may not include an "expected" recently-used record despite the record being recently accessed. For more information about how blank Lookup searches operate, please review Standard and Enhanced Lookups in Salesforce Classic.
An org has over 10,000 records that start with the word "Spanish." A search for "sp" returns a maximum of 250 records, and then the Lookup Filters evaluate those results according to the filter criteria. If the 250 records returned in the initial query do not meet the criteria of the Lookup Filters, the lookup search returns 0 results — making it appear as though it is not working as expected, even though it is.
Consider promoting the Show all records when selecting a record from a lookup field Idea on the IdeaExchange so our product team can see your feedback about this functionality and evaluate it for improvement in a future release.
Standard and Enhanced lookups in Salesforce Classic
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