It is unusual that the filter does not work with the operator EQUAL for a single-word value in the modern News rollup with a correctly indexed custom property. The operator EQUAL should be able to accurately match a single-word value.
There could be several reasons why you are experiencing this issue. Here are a few possibilities:
Indexing Issue: Ensure that the custom property is properly indexed in SharePoint. Check the indexing settings for the property and ensure that it is set to be searchable and queryable.
Case Sensitivity: Check if the operator EQUAL is case-sensitive for your custom property. It is possible that the value needs to match the case exactly for the filter to work. Try using the appropriate case when using the operator EQUAL.
Data Format: Verify that the single-word value in your custom property is stored in the expected format. There might be additional characters or spaces present in the value that are causing the filter to fail with the operator EQUAL.
Configuration Issue: Review the configuration of the modern News rollup and the custom property. Check if there are any other settings or conditions in place that could affect the filtering behavior.
This behavior should be the same for metadata fields in SharePoint. Whether it is a custom property or a metadata field, the operator EQUAL should be able to accurately match a single-word value.