If you have a choice column in SharePoint, where you have enabled "Allow Fill-in values",then Power Apps will not respect that setting. For that to work you need to update the Datacard for the column to use the search text as input if nothing else is selected in the choice list.
To get this to work, you need to:
- Go to the app and find the dataCard og your Choice column. Here you need to set the Update property to: (On my demo, the card is called "DataCardValue2")