When using Advanced Search with a number type custom property and the between operator, two issues are observed:

The filter ignores the specified range and returns all entities that have the custom property set — regardless of whether the value falls within the range. For example, filtering between 1 and 5 returns an entity with value 100.
Sometimes when the between operator is selected and values are entered, the search API is not triggered at all — the results remain stale from the previous query.