If you use SharePoint as data source in Power Apps and you have dropdown fields for lookup fields, they’re only searchable as “begins with” but not for “contains”.
This is caused by SharePoint, if you use the default Items value Choices().
You can get around this problem by building a collection.
The only trick, that also the update to SharePoint works, is that the collection may only consist of the columns “Id” and “Value”. You can achive this with the following formula:
Now use the collection in the dropdown Items property.